How many hosting accounts would be considered ok without over selling on a 30GB VPS with 300GB bandwidth?

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?

1024MB of RAM

I have all the processor speed I need since it's running at less than 1% and I have 3 domains on that server. The cpu last I checked is showing 0.7% usage.

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.

I mean Quad core, Dual core, Dual Quad? Speed, cache etc.

Then ram wise, it depends, how much each site will consume etc, it cannot be calculated at all, only estimated.
I have 2 servers (Quad core), managing about 600 sites in total, and the speed is fine, they cope on 4GB ram each, 4 x 2.67GHz, 8-12MB cache.

However one site can slow down the whole Node, it is one of those things were you need to experiment to find out.

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 1000MB and 10,000MB of bandwidth. There is still a limit as to what they can do. So my thinking is that with 30GB disk space and 300GB bandwidth on the machine it's reasonable to think that 30 hosted accounts could be placed on there. Does that make sense?

If you have 30 hosted accounts using 500MB web space and 5000MB you are consuming 50% of your resources which leaves room for 15 hosted accounts using 1000MB of web space and 10,000MB of bandwidth which will consume the other 50% of your resources. So in this case 45 hosted accounts would fit on the server.

Now, my question is, should I allocate all that space based on the bandwidth I have total or should I under sell slightly so that there is no chance of using 100% of the server resources? I think some hosts try to squeeze in more thinking that not everyone will actually use 100% of the resources allocated to their hosted account.

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.

Yes, I understand architecture. I did go to a tech school several years ago. But if the cpu and ram are fast enough to handle all that is on the HD then it is possible to use a good portion of the space and bandwidth. I do hear what you are saying and I've been aware of servers that only have 256MB of RAM and they crash with only 3 domains on them.

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.

Yes, if you read my posts I already stated that. The most I expect is maybe 45 and mostly what I am wondering if that is feezible since it seems like it is, according to the math I've done on it.

well ,, if you are really trying to comparison between number of hosted accounts and the server configuration you should post the RAM of the VPS too .. but again it doesnt really depends on the server set up ...that how much hosted accounts can be on a so so GB of disk or thing like it