Probably 5, one in the middle and two in each "fort". If you're using the TFC/TF2 map as a template, Firefight's non-linear checkpoints allow for the most traditional gameplay, or if you're making the map really long, you could do Battle and have shifting spawn locations.
The one concern I would bring up for Firefight style battles is that with 2 cap points inside the enemy base, it could make it rather difficult for any one team to capture and hold both points long enough to win.
Conversely though, for a Battle Map, unless the map is large (long) it seems like the map wouldn't be big enough to support spawn changes or would unbalance the game in favor of whoever captures the first point.