You only need the one license, but you must use the same email address that you used to purchase MC. If you save your (restore) keycode to a floppy or something, then you can restore from that on any machine you use for a 2-week period, (you get 10 restores a year). The only restriction I know of is that you can't run more than 1 instance of MC on the same machine concurrently.
As far as prices go, only based on passed experience, when v12 starts in it's beta form, it'll probably be similar to the previous betas, about $10-15 and shortly before release around $25-30, with the first-time purchase being around $40.These are just guesstimates.
In any event, MC 11 has just been released, so it will be a long time before we'll see MC 12 Beta.
As far as I know there have never been any "security" fixes, however anything critical to the functioning of any MC version has generally been fixed in a timely fashion. If not critical, it will be addressed in the next version. There will no doubt continue to be some updates to v11 before J River moves on to MC 12.
Media Center is an active ongoing project as you can see if you monitor this forum a bit. It is because of this two-way communication that MC has become the best media product around.