How do I understand how much system resources I need for a certain project?

Let's say I have to host 25 websites.. How do I know how powerful should my VPS be? Which specs it should have, how fast the connection should be to handle X visits per day?

How do you understand which are your system requirements BEFORE deploying a project? Do you just make estimates and then scale up? Or there's some kind of tool to benchmark? how to handle this kind of stuff?