1) How easy is it to do a multi monitor setup with HUD on one and say instruments view on the other? Do I need 2 3D video cards or can a single dual port 3D card handle it?
Not sure this is possible at all, but I have never tried it.
2) Does LB2 need a CD key to play online? And how big is the multiplayer community? (any place where I can see online player stats?)
Nope. No CD key or anything necessary. It runs purely as peer to peer. There are no intermediate servers, and hence, no stats of anything other than what is saved on your machine.
3) Maximum number of players in a multiplayer game?
I think this number is 8 for all practical purposes, but might be higher. There are 8 available choppers in any mission (4 flights of 2) and though each chopper has 2 positions (Pilot/CPG in the AH-64, Pilot, Navigator in the OH-58, Pilot/Doorgunner in the UH-60) I don't think you can fill them all in a MP game. The MP community is pretty small, so getting 2-4 together is hard enough. Something that somewhat limits MP is the problems with hosting through a NAT router. Numerous threads here describing this. HAMACHI seems to work to overcome this.
4) Do we encounter any enemy aircraft? (specifically air superiority fighters liek F15s, Mig29s)
Enemy Mig-29's will bounce you often and are pretty deadly. Often the the time between detection of a MiG and being burned down with a missile is incredibly short...but with sufficient cover, and quick reactions they can be downed with Stingers.
im wonderign the same things about EEAH and the other EE one.
The answer is pretty much the same for EECH and EECH2 (don't bother with EEAH other than as an addon to EECH). EECH has more varied aircraft types to encounter. I think the max is 16 for EECH players, but again, it is rare to get more than 2-4 together at one time. EECH has a 'join any time' type connection where players can join and exit a server throughout the course of the campaign/skirmish. LB2 requires everyone to connect at the same time.
ps - i Love being able to have multiplayer Pilot- Co Pilot in a helo. I wanted to do it with F15 Strike Eagle II but of course could never find anyone. Having a second human do ground attack would relieve a lot of the load on the pilot who has to fly Air-to-air.
We recently managed to connect MP in F-15 Strike Eagle III via DOSBOX in both Leader/Wingman and Pilot/WSO mode, but performance was pretty poor.