The normal version can monitor only the server it's installed on. The Plus version has alerts and you can use it to monitor as many servers and apps as you like - there are no internal restrictions besides the memory of the VPS it's installed on. So if you want to monitor 10-15 servers - you will manage with a 128-256MB VPS, if you have more than that, all you have to do is to upgrade it to 512MB RAM or more.

So, is that something where the "central"/plus server polls the other servers that are running the normal Amon logger/monitor for their info then just displays it in one place?

It's the other way around. On the servers you install a 10KB agent that collects the data and sends it to the central instance

I think the main extra feature is the ability to monitor multiple servers in a single place.