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?
That question is going to be impossible to answer without a lot more details. The number of websites is largely irrelevant (each website will use a negligible amount of RAM for the web server process to know about). What you want to know is the total number of HTTP and HTTPS requests per minute (the latter being a bit more expensive) in peak times to estimate the required CPU horsepower, the amount of data transferred (network bandwidth and CPU to some degree), whether it will be mostly static pages or dynamic/scripted content (CPU and RAM), and of course disk space to store everything (a stock photo library will likely use more space than a pizza place).
If there’s a database backend you’ll want to add even more RAM and faster storage (both in terms of throughput and IOPS).
Also, acceptable waiting times. An under-powered server will work just the same, just slower.
If you know a bit about the websites you want to host but need some pointers, maybe start by checking out some packages by other hosting providers (how much CPU and RAM does their ‘local chess club WordPress site’ package offer?) and go from there.