How many users do you put on a single server? What are the server specs? Why have you chosen to do this? In your opinion what is a stable server load for hosting shared account clients and when should you start to make plans for new servers?

it depends. If they're all running forums like this then not so many per server. If just static pages you can cram a lot more in. If you start using more than 50% / 60% ram & having a high server load you know you need to spread them out a bit.

I have a red line at 80%. Whenever any of my used resources (RAM, transfer, storage, etc.) reaches 80% of the maximum capacity, I upgrade or get a new server or something.

When my server load goes above 3.00 I get a new server. I can fit about 175-200 clients on each of my servers.

It all boils down to load balancing the servers often. A server running 100 websites with no problems can change 2 months later when a few sites start getting really popular and drawing more resources. If a host does not monitor these things, they will surely run into trouble. We just run dual xeons and you can pack quite a few accounts on them but I refuse to do it. I am not that money hungry and I also want to keep my clients happy.

Depending on your server, you can typically fit 200-400 users, and being hesitant on the higher end. Just depends on how well your servers are managed in the end

I honestly don't think you can place a number on things. I have one reseller who accounts for 50% of the sites on one of our shared servers.

I just noticed my server load was starting to get high, so I upgraded my RAM today

Since my servers are really cool (and highly clocked - thanks to freezer cooling, and biiiiig air conditioners) and they are really stable, I can fit about 200 clients and not be near 80% load.. My RAM on each server runs at about 5Gb, crazy I know, but the cooling systems can just clock it up and all, I also share data conections with all my other datacenters, so that helps, the clients data just floats in the middle of cyber space kind of thing, hard to explain unless I could draw a picture..

It really depends on the server specs, but I tend to put around 100-200 users per server.