Ive never seen a total player count but it's nearly always 12 v 12. I read on another post it's the top 7 (I think) XBL game too. It's not hard to get used to, should play the campaign first to get a feel for it. When you start multiplayer make sure you join a squad and I guess try and follow them if you can, it'll help learn good routes to M-Comm stations etc. You'll also be able to respawn off them to save running miles back to the action. Read other topics too there are a lot of helpful people on this board and it might help you out some more.
if you've been playing a lot of CoD, the learning curve could seem a little steeper as games last quite a bit longer than you're average CoD match and the gameplay is slower compared to Duty. That being said, teamwork is required if you want to win (nothing worse than joining a squad with 3 snipers, not recon, but snipers). Knifing is what took me the longest to get used to, you actually have to pull out your knife to stab and for 360 users it is the RB not the RS. Can't tell you how many times i died attempting to knife someone when instead i just crouched in front of them giving them an easy kill. There's really not much camping (other than snipers which is fine with me)that goes on because maps can be huge and nobody really concerns themselves with K/D. To sum it up, I like CoD but I love Battlefield...it just seems so much more rewarding. And it will take forever to get to level 50. I have played for close to 160 hours and I am a 35.