How to correctly calculate server needs based on the number of active users?

Question

Server equipment

I don’t know how to roughly calculate what kind of server I need (with a volume much less clear), is there any calculator, etc. The application itself will be on the server in java, mysql, ..., the main purpose is to issue news, a daily audience Let's say 100 thousand unique (well, for example). I have no idea how to calculate the more or less necessary configuration, but I don’t want to buy and update servers depending on the influx of users

Answer 1

and you don’t want to buy and update servers depending on the influx of users

And it will have to.
Tests, tests, more tests.
Takes 1 computer, drive traffic there and look at the indicators and draw conclusions.