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If I create my social network or forum, how many users can my social network or forum have?
Ah, that's the magical question of any hosting service or server: how many visitors can my website have.
The answer is that it's impossible to predict. It depends on:
These are just the factors I could come up with now, I don't doubt that there are plenty of other factors which are relevant but I couldn't come up with right now.
And even if you have detailed, exact numbers for all of these questions, it's still impossible to boil them down to an amount of server power per user.
The only way to figure out how much users your website can handle is to open it up for users and measure how you're doing resource wise. Maybe it's 10 users, maybe it's 10 million users. There is only one way to find out.