When BIS were testing the 1.88 server patch on the server we had a heap of people joining and we reached well into the 40-50 mark but not 64.
Lag can be caused by the CPU simply unable to process that many network instructions for that many players, and also the server's available bandwidth would be stressed dramatically.
The facts are that OFP has a much larger play area, and more data needs to be sent from server to player, and back again.
Other games, that are more simple, that have a small play area, no vehicles etc, simply need to send a smaller ammount of data, e.g player location, what gun he has, what direction he is moving, etc. So they can support more players on less CPU and less bandwidth, but they compromise heavily for that privilege as you know.
Now BIS have reworked network code, I think they have done all they can with the existing engine, so if you have a powerful server, at least 2.4ghz, and a nice chunk of bandwidth for the server, 32players is not a problem, or indeed maybe you can get as high as 48.
OFP2 I think they are tackling it from another angle, with ideas like "terrain streaming" and stuff but I think we will need to wait and see what BIS come up with.
I trust whatever it is it'll be damn fun, can't wait