Is there a console command to unlock factory slots? Looked through the list at the wiki and couldn't find one.
The best way is unlock all researches about construction.
Remark : If i am remember correctly you could also change the number of slots about each type of province but AI will be affect too.
Do you mean, a cheat to give max building slots per region, so you were not limited to 2 or 3 in some regions?
Yes