Can the code I wrote support 100,000 concurrent users?

Honestly, there's no simple "yes" or "no" answer to this question. It's like asking: "Can my restaurant serve 500 guests at once?"

The answer depends on many factors, not just how good the chef (your code) is.

1. Your Code (Cooking Skills and Recipes)

Code Efficiency: For the same dish, a skilled chef (efficient code) might finish it in 5 minutes, while a less skilled one (inefficient code) might take 15 minutes. When there's only one guest, an extra 10 minutes isn't a big deal. But when 100 guests order the same dish simultaneously, the cumulative extra time becomes terrifying, and later guests will wait forever and a day.
Resource Consumption: Is your code particularly "resource-hungry"? For example, does making one dish require an entire bottle of oil (consuming a lot of memory)? Or does it need to occupy several stoves simultaneously (consuming a lot of CPU)? If it's too "expensive," your server's limited resources will quickly be depleted.

2. Your Server (Kitchen Size and Equipment)

Hardware Configuration: You can't expect a small home kitchen (low-spec server) to produce the volume of a five-star hotel banquet. CPU is like the number of stoves, memory is like the size of the prep counter, and bandwidth is like the size of the serving window. 100,000 concurrent users place very high demands on this hardware.
Database: This is your large refrigerator and pantry. If your fridge (database) is disorganized, and it takes ages to find a tomato every time (slow queries), that's definitely not going to work. And if 100,000 people try to rummage through the fridge simultaneously, they might just break the fridge door off (database crash).

3. Your Architecture (Restaurant Operating Model)

Monolithic vs. Distributed: Are you running a small food stall where the owner, chef, waiter, and cashier are all you (monolithic architecture)? Or are you running a large restaurant with dedicated cold dish, hot dish, and pastry sections, plus many waiters and cashiers (distributed/microservices architecture)? When there are few people, one person might manage. But with 100,000 people, you'll definitely collapse on the spot. Only a large restaurant model can distribute the pressure.
Caching: For some popular dishes (hot data), like "Today's Special," can you prepare a large batch in advance and just serve a portion to whoever wants it (using caching)? Instead of cooking a fresh one for every order. This can greatly reduce the pressure on the chef and the kitchen.

A Key Misconception: "Concurrent Online" Does Not Equal "Concurrent Operations"

Out of 100,000 users "concurrently online," perhaps 99,000 are just idling on the page, and only 1,000 click the "buy" button in the same second. You need to focus on this latter number, which is the "concurrency." This is what truly impacts your server. It's like a restaurant full of 500 people, but at any given moment, only 50 tables simultaneously call out, "Waiter, we'd like to order!"

Conclusion: How to Know if it Works?

Don't guess, test.

In the IT industry, we have something called "stress testing." It's like before a restaurant opens, you gather a bunch of relatives and friends to pretend to be customers, flooding the restaurant at the same time, and trying to challenge you in the most difficult ways, to see where your kitchen, waiters, or cashiers will break first.

Through stress testing, you can identify your system's bottlenecks: Is the code too slow? Is the server configuration insufficient? Or are there issues with database queries? Only then can you optimize specifically.

So, don't get hung up on whether your code can handle it "right now." The more important question is whether you have a plan for how to gradually upgrade your "kitchen" and "operating model" when user numbers grow from 10,000 to 100,000. That's the key for a startup to survive.