To be honest - this is a very important one =
a city with city size 1-2 should only field one new brigade each turn,
a city with city size 3-5 should only field 2 new brigades each turn,
a city with city size 6 or higher should only field 3 brigades each turn,
you can also make this condition even more hard = only a new brigade can be introduced if all present brigades have been filled and ready for action (red banner units are gone).
Why ? Well - Tranquill and his Bushwackers ride up North and take one city in Missouri - the next turn I order 8 brigades from Missouri (reg and irreg) and they all arrive in this one city. This is a behemoth and totally ahistorical and gamey. From there on, the Union has a big problem.
If Tranquill is able to capture one city - it's a little naive to assume that a few weeks later - 8 confederate brigades are present in this one city!
Off course, if you have more cities in a state, there's no problem no more, the newer units are spawned in cities that are not "growing" a new brigade.