How many websites may i set in a vps?
what are requirements to run about 30
That depends entirely upon the sites as Chris S. noted - if you are trying to do things as cheaply as possible and you are comfortable with systems administration, I would recommend the 64MB VPS Static Webserver guide at LowEndBox.com ... if you know you'll need Apache and MySQL, you will probably be looking at needing at least 256 MB of RAM as a bare minimum (again, assuming you're comfortable with systems administration).
May i set my server, for example, to
use apache + red5 ?
A fully-managed hosting service should include systems administration assistance to install whatever you need - this is the recommended option if you are not comfortable administering the VPS yourself.
An unmanaged hosting service will drop you at the bash prompt. If you're not comfortable using bash, you probably shouldn't use an unmanaged hosting service (it will end up costing you a lot more time and money in the end).
Can you suggest some good resource to
understand better the VPS and how to
If you need to learn Linux systems administration (and you have plenty of time on your hands) the first command you need to learn to use is man
As far as VPS hosts go, you might try shopping around at comparevps.com to get a feel for what different providers offer and how much you can expect to spend.
One thing you may want to consider (assuming you are looking at Linux hosting) is that OpenVZ virtualization makes overselling easy - it's comparatively difficult to oversell with Xen virtualization, so go for Xen (particularly if you plan to run Java).
May i resell shared hosting in my vps?
Read potential providers' Terms of Service. Note that many providers offer cPanel licensing for a nominal fee (if you plan to offer website/e-mail hosting, cPanel will make your life a lot easier).