The answer it: it depends.
If you have a simple site that’s only a few pages of (mostly) static HTML, you may be able to have tens of thousands of visitors per month. If you have complex software running on it with a lot of interactions, a few people may be enough to overload it.
It’s pretty much impossible to predict it. All I can recommend is to just try it and watch the graphs. You can always upgrade or downgrade later on depending on how things go.
And in general, spend how much you’re willing to spend, but also don’t optimize for millions of visitors when you’re just starting out, because then you’re just wasting money.
50,000 hits per day, not per month. So by following KangJL’s math:
That would give you a maximum of 30k page views per month, which would be enough.