From 0 to 100k Users: The Ultimate Tech Stack for Scaling a SaaS in 2026
There is a famous saying in the startup world: "Premature optimization is the root of all evil." This means you shouldn't worry about handling 1 million users when you only have 10.
But in 2026, there is a new reality: "Technical Debt is the silent killer of growth."
We see this tragedy happen all the time at Rehmall. A founder builds a cheap app using a "No-Code" tool or a messy spaghetti-code setup to save money. Then, suddenly, a TikTok video goes viral. They get 10,000 signups overnight. And then? The server crashes. The database locks up. The app becomes slow. Users get angry, leave 1-star reviews, and delete the app. The opportunity is lost forever.
Scaling isn't just about adding more servers; it is about choosing the right Architecture from Day 1. You don't build a skyscraper on a foundation meant for a wooden shed.
In this guide, we are going to act as your Fractional CTO. We will walk you through the exact Tech Stack (Next.js, Supabase, Flutter) that allows you to go from your first user to your 100,000th user without rewriting your code.
1. The Database: Why SQL (Supabase) Wins the Scaling War
The heart of any SaaS is its database. If the heart stops, the body dies.
For years, startups used Firebase (NoSQL) because it was easy. But as you scale, Firebase has a major flaw: Data Duplication. In NoSQL, finding "All users who bought the Premium Plan AND live in New York" requires complex, expensive queries. As your data grows, your Firebase bill explodes, and your app slows down.
The 2026 Solution: Supabase (PostgreSQL)
At Rehmall, we build scalable apps on Supabase. Supabase is built on PostgreSQL, the world's most advanced open-source Relational Database.
Why it scales to 100k+ Users:
Relational Power: You can connect distinct data points (Users, Orders, Products) efficiently. SQL is built for complex queries.
Vector Search (AI Ready): In 2026, you will likely need AI. Supabase has built-in "pgvector" support, meaning you can add AI search and recommendations directly into your database without buying a separate expensive tool like Pinecone.
Row Level Security (RLS): Security is baked into the database. You define rules like "User can only see their own orders," and the database enforces it. This prevents data leaks even if your frontend code has a bug.
The Verdict: Firebase is for hobbies. Supabase is for businesses that plan to grow.
2. The Frontend: Next.js 16+ and the "Edge"
When you have 100,000 users, speed is money. Amazon found that every 100ms of latency cost them 1% in sales.
If you build a traditional React app (Client-Side Rendering), the user's phone has to do all the work. If they have a cheap Android phone, your app feels slow.
The 2026 Solution: Next.js (Server-Side Rendering)
We use Next.js 16+ because it shifts the heavy lifting from the user's phone to the Server.
The "Caching" Secret
Imagine 10,000 users visit your "Pricing Page."
Old Way: The server builds the page 10,000 times.
Next.js Way: The server builds the page once, saves it in memory (Cache), and serves that snapshot to 9,999 users instantly.
This reduces your server bill and makes the page load in milliseconds. Next.js is the only framework that handles this complex caching automatically. This is how sites like Netflix and TikTok load so fast.
3. The Mobile Strategy: Why Flutter is the Only Scalable Choice
When you scale, you will face the "Update Nightmare." You find a critical bug. You fix it on the website. Now you have to fix it on iOS. Then fix it on Android. That is three separate codebases. By the time you fix it everywhere, you have lost users.
The 2026 Solution: Flutter
Flutter allows us to maintain One Codebase for Mobile, Tablet, and even Desktop apps.
Does Flutter scale?
Ask Nubank. They are the world's largest digital bank with 90+ Million customers. Their entire app is built on Flutter. Ask Alibaba. Millions of transactions per second. Flutter.
With the new Impeller Engine (released fully in 2025/26), Flutter apps now perform exactly like Native apps. There is zero lag. For a scaling startup, Flutter means you only need One Team to manage your iOS and Android apps. This cuts your "Burn Rate" (monthly expenses) in half, allowing you to survive longer.
4. Infrastructure: Vercel vs. The "Custom VPS" Savings
Here is a secret most agencies won't tell you. Hosting platforms like Vercel are amazing for getting started. They are easy. But when you hit 100k users, Vercel gets expensive. Their "Bandwidth" costs can eat your profits.
The Rehmall Scaling Strategy:
Stage 1 (0 - 10k Users): We host you on Vercel. It is free/cheap and zero-maintenance.
Stage 2 (10k - 100k Users): When the bills get high, we don't rewrite the app. We simply move the Next.js app to a Custom VPS (like AWS or DigitalOcean) using Docker or Coolify.
Because we write standard Next.js code, you are not "locked in" to Vercel. We give you the freedom to move to cheaper infrastructure ($20/month VPS instead of $500/month Vercel bill) when you become successful.
5. The "Microservices" Trap: Keep it Monolithic
A common mistake founders make is thinking, "I need Microservices like Uber!" They build 10 different small servers (Auth server, Payment server, Email server).
Do not do this. Microservices are for teams with 500+ engineers. For a startup with 100k users, a Modular Monolith is faster, cheaper, and easier to debug.
We build your backend as a single, powerful unit (using Next.js API Routes or Supabase Edge Functions). It is simple to deploy and simple to scale. If one part needs more power, we just increase the server size. Simple is scalable.
6. Monitoring: How to Know Before it Crashes
Scaling is blind without eyes. You cannot fix what you cannot see. In our scalable stack, we integrate Sentry and OpenPostHog.
Sentry: It tells us "User X in London faced a crash when clicking the Checkout button." We see the exact line of code and fix it before other users notice.
PostHog: It tells us "Users are dropping off at the Sign-Up screen." This helps us optimize the UX to handle more users.
7. Summary: The 2026 Growth Stack
So, what does the perfect 100k-user architecture look like?
Database: Supabase (PostgreSQL) - For solid, relational data.
Web Framework: Next.js 16+ - For SEO and caching speed.
Mobile App: Flutter - For consistent UI across iOS/Android.
Hosting: Vercel (Start) -> AWS/Coolify (Growth).
Payments: Stripe - The standard for handling millions.
8. Conclusion: Build for the Future
Scaling is painful, but it is a good pain. It means you are succeeding. However, if your technical foundation is weak, that success can destroy you.
You don't need to hire a Silicon Valley CTO who demands $200k/year. You just need an agency that understands Architecture.
At Rehmall, we don't just write code; we engineer systems. We build apps today that are ready for the millions of users you will have tomorrow.
Don't build a toy. Build an Empire. Let’s discuss your scalable architecture today.
Start scaling with Rehmall: https://rehmall.com/services
Frequently Asked Questions (FAQ)
Q: Why not use Python/Django or Node.js/Express? A: You can, but Next.js offers a unique advantage: it handles both Frontend and Backend in one framework. This reduces context switching for developers. Supabase handles the heavy backend logic (Auth, Realtime), so you don't need a heavy Django server anymore. It keeps the stack "Lean."
Q: Can Supabase handle millions of rows? A: Yes. Supabase is just PostgreSQL. PostgreSQL handles petabytes of data for huge companies. It is battle-tested for 30 years.
Q: Is Flutter fast enough for 100k users? A: Flutter performance happens on the device, not the server. So, having 100k users doesn't slow down the Flutter app; it slows down the backend. Since our backend (Supabase) is scalable, your Flutter app will remain silky smooth regardless of user count.
Q: How much does this "Scalable Stack" cost to build? A: Surprisingly, building scalable architecture doesn't cost much more than building bad architecture. It just requires knowledge. At Rehmall, our pricing remains affordable because we use efficient tools, not because we cut corners.
Q: What if I am already on Firebase? Can you migrate me? A: Yes. We specialize in Firebase-to-Supabase migrations. We transfer your users and data securely, usually resulting in a significantly lower monthly bill and faster query performance.