As mentioned many times before, this totally depends on the usage of those sites. You can be hosting 100 sites or 10 sites (even though you offer just 1 database). Although it most likely won't be highly visited sites, depending on your bandwidth (and considering that you wouldn't oversell), I'd say you could put around 50 - 80 sites (and that is if bandwidth limit is more than 200 GB - if you choose not to oversell).
Also, what OS are you looking for? Linux or Windows?
As many as you can Just put people on there until the loads reach a certian level. I normally stop adding when loads are averaging are around .60-.80. Remember if you are selling reseller account give the resellers some room to grow as well.
Here's something to think about...have you thought about how you're going to provision this server w/o a cpanel? I'm sorry...I had to setup up a server without any kind of control panel *once* and that was enough for me. Thank goodness there are free control panels!
Oh! This new one (to me) is ready go for debian. You might want to take a look: