There will be 60,000 slots available and only one map for testing -- Sewers, according to a dev blog post, which is said to feature "an interesting mix of outdoor and indoor, vehicle and infantry combat, deployables and counter-deployables and good old-fashioned FPS combat." It's likely the map with the most possible situations for bug-testing. All slots will go to FilePlanet users, with half of the slots set aside for those who pay for the premium subscription.
The post makes it very clear that this is not a demo. "If you want to just play a game for free you may find it a frustrating experience," it said. It also cautions that the beta will be taking data about your hardware configuration, "so you shouldn't apply unless you're comfortable with that."