The most I've had at once is 4 - that was definitely daft and walking became a chore....
My current 3 are little, big, little, ages 11, 3, 2, boy, girl, boy.
If your first two are pretty much fully trained then a new dog will fit in OK with some hard work. Unless you are taking them out separately all the time - you need eyes in the back of your head to manage poo pick ups, recall, horse manure, long lines, for each of them.
IF either of your older dogs is problematic - poor recall etc - then you might be in trouble. I'd work hard to make sure the basics, like recall, were VERY good with the existing pair and then think about a pup.
I was doing advanced obedience with my girl when I got the youngest at 6 months (after his owner died) and his recall was "haphazard" - she was brilliant at going to "retrieve" him from the other side of the park - she'd run over, engage him and he'd run back with her. She'd do it on command too 😎
It depends a bit on breed, but I've found bitches have a less dramatic teenage phase than the boys - they seem to settle down more quickly.