Hari I think it’s impossible to give you an reliable answer here. Flarum/core is really lightweight anf does not at all use a lot of resources. Still it’s highly depended on how your server is setup and how your unsers are interacting.
Then the extensions come into play. Those can highly impact the performance of your forum.
So in conclusion. Best to start small and monitor constantly. With today’s virzualized infrastructure on most hosting offers you can upgrade to a higher tier giving you more resources when needed.
I hope this helps you although it might not exactly answer your question.
