15 points is the total for your starting list (Build Points and Base Points = 15). Once the first game is over (whether you can re-equip or not and whether or not you have gained any skills), the Base Points have a limit of 15 for the List. You can take as many Build Points as you wish, remebering that the more Build Points that you take, the more your list's total points value, and hence the stronger (and harder to kill) Lurkers that they will face.
Special Agents add to your list's Build Points (and total points), but do not count towards your 15 Base Point limit. This way, when your list is max'ed out and/or your character is at a 15 point base (if he/she lives long enough to see the day), you can still add Special Agents.