It is never based on disk nor bandwidth, well it is, only on how much space to sell.
It more or less depends on ram and CPU, this is the first and most important to learn.
Maybe tell us the CPU and what ram you have?
When you have less then 20% free space, time to add more space.
That's not an issue at this point. I have lot's of space, 80% approximately. I'm trying to discern what is a good rule of thumb to use as to figure out how many domains I can host with the space I have. Even though my plesk on this particular server is licensed for 100 accounts I doubt I could actually put that many on the server without bogging it down and maybe even shutting it down. I know that when the server runs out of RAM it will cease to function.
How many hosting accounts would...
Depends entirely on the site, users/visitors, technology etc.
maybe none, maybe 50 - there's absolutely no way to be certain based on just the server specs.
what are the target clients running ? scripts ? forums ? blogs ?
how busy are they ?
what technologies ? php ? java ? ruby ? perl ?
they're the sort of questions you need to consider, as well as constantly reviewing the vps performance/setup/security
Let's say on average a hosted account is...
You're still not grasping that as a metric for working out how many sites you will host, disk and transfer are largely meaningless - in lmost 20 years of providing hosting services, I've never seen a shared-hosting server run out of disk allocation before hitting other limitations.
You would not host 100 shared accounts on the VPS, unless your customers do not use the allocated resources and do not consume the allocated CPU. As you know everything depends on the resources usage and it is really hard to generalize just like that, without having usage statistics.