Some great information here: http://www.gamespot.com/articles/how-battleborns-rpg-mechanics-extend-its-replayabi/1100-6435511/
With seven starting characters and an additional 18 to unlock, Battleborn’s RPG elements hint at a longevity sometimes absent from multiplayer shooters. The extensive web of unlocks, items, improvements, and challenges are enticing, and could be the thing to set Battleborn apart in the multiplayer shooter landscape when it releases on May 3.
So, while I am sure that is subject to change, as it stands, we start with seven Battleborn unlocked.

