That’s impossible to say. It could break at one user or support tens of thousands without any problem. There are way, way, way too many variables related to how the software works and how your users interact with it to make any sort of guess.
All you can do is open it up to users and watch what the resource usage graphs do.
For starters, InfinityFree doesn’t have paid plans. Like the name implies, we only provide free hosting. iFastNet provides the paid plans.
But the answer for the paid services is exactly the same as for free hosting:
Lacking suitable information here are a few recommendations:
How much are you willing to spend? Establish a budget for yourself. Then get a service just within that budget and see how it goes. You can upgrade or downgrade down the line as your needs become more clear.
Is this an existing project? If so, please provide details about the current user numbers, the current hosting you use and what is and isn’t working well for you. And how do you expect the usage to change in the near future? We may be able to recommend you something more easier, more powerful or more cost effective than what you have now.
Is this a new project? If so, start small and simple. Establish what components you need to go live and welcome your first users, and get a service that can provide those. Being ambitious is good, but getting servers to handle 10,000 users while you don’t even have 10 users is just a waste of money.