Welcome to the loony bin -- I mean, the forums!
I don't know the exact number of maps. Since single-player and multiplayer are intertwined, the campaign levels and multiplayer maps are one and the same. So I'd guess a healthy number of maps (10+) will be available for online play at launch.
I haven't heard any
official word on DLC, but the devs implied they'll be supporting Brink after launch and will be re-evaluating many aspects of it after it ships.