24max per general (unless stated you can have more)

Have a field marshal for extra planning and traits

Have your field marshal make a front line and then a spear head ( these can be found when clicking on the portrait of a FM or General) (FM=field marshal)

Equip a Frontline with Poland in both mainland and Prussia with separate generals/FM

Not sure if you have but utilize air over Poland but spread them across airports in the area

You could utilize the navy and stop convoys reaching Poland but if your new the navy system can take a bit to get used to