First run through. Not sure of were you find the unit build limit. Is it on the faction sheet. Example of rules states Arcturus Mengsk build limit is 2. Cant see were you find this. Many thanks in anticipation.

Here's the exact quote:

Building Units
If the active player has a base on the active planet, he can
build a number of units equal to his unit build limit . The
starting unit build limit for the Terran and Protoss factions
is two, a number that can be can be increased by purchasing
Supply modules (see “The Supply Module” on page 40).
The Zerg unit build limit works differently, as explained
on the Zerg Faction Sheets and under “The Zerg Unit Build
Limit” on page 40.