At Standard Ebooks we serve a respectable number of page views and ebooks each month - and have been on the front page of HN three or four times - all of it done with a single 4GB VPS. And the only reason we upgraded to 4GB from 2GB is because we needed more RAM for the server to build the extremely large Decline and Fall of the Roman Empire ebook - if it weren't for that, our 2GB server would still have been just fi…
But let's be clear, your serving infrastructure is able to be that simple because you outsource donation management to https://fundraising.fracturedatlas.org, contribution management to https://github.com/standardebooks, collaboration, membership and communication to https://groups.google.com/g/standardebooks, and marketing signup management to... looks like postmark.
So sure, you can run your stuff on a single server, but you're relying on a bunch of other people running much more sophisticated services on a lot more infrastructure in order to do it.