@decipher it seems like you’re trying to find the perfect recipe to start a successful shared hosting provider. The truth is, there is no one size fits all solution. You should research commonly used panels, pair these trials with various server offerings and see what works best for you. It’s impossible to give an exact number on the amount of users who will fit onto a specific server, since different users with different use-cases require different resources. This is where resource limiting comes in, but that’s a bit more technical.
Cpanel is the standard and has been for a long time. However, it’s highly probable that this will change due to their recent pricing structure change. DirectAdmin has been receiving a lot of praise since Cpanel’s pricing announcement, and has received a lot of quick development thanks to the heightened awareness.
If you are looking to start a hosting company, personally I would advise you to simply use reseller hosting provided by someone like BuyShared. They’ve done the research, they’ve got the technical expertise, and there isn’t much money in shared hosting unless you really know what you’re doing. This is just my opinion though, and I wish you the best of luck with whatever you choose to do