just a stab in the dark here since I dont realy know much about this but if you have done any carving you may have more faces then you need try to consolidate

Yes theinquistor pointed out if you carved anything you probably created a ton more brushes then you should have.

As far as that error goes it means you have way to many brushes in your map and need to scale back by either converting some to props using proper or just getting rid of them altogether.