Greetings!
The goal limit is controlled by the server cvar
tf_flag_caps_per_round, which is the same one that controls the flag capture limit in CTF. Unfortunately this is a server cvar, and the only way a mapper can change it is by using a point_servercommand (which are frowned upon because of security concerns and are disallowed by default), or by including a config file with your map as a separate file.
The reason such things are controlled via server cvar rather than by the map is to give the server operator more control over the experience he offers players. I know that can get in the way when you are trying to make an unusual version of a game mode.
The people working on Pass Time are receptive to feedback. You should ask in the
Official PASS Time Steam group about letting the mapper change the goal limit in the
passtime_logic entity. I advise against taking this control away from the server operator, though.