It is highly individual.
The thing is, your dogs are young. 16 months and 9 months and even the border collie is a youngster. Adding another young'n, well, the thing is, your dogs are mature yet. As dogs mature, their personality starts setting, and some will find living with dogs of the same sex less tolerable than others. You might find that one of the shepherds is dominant at this point, but give another six months and that baby might decide that it is the top dog in the pack.
And as your pack grows, the chances that nobody is going to have an issue with a newcomer gets less and less.
Too many dogs is the number that you find you can no longer do as much as you would like with each one.
Too many might be when the cost of maintaining them, training, vet, grooming, etc, becomes a strain.
Too many may not be four today, but 10 years from now when the dogs are between 10 and 12, and senior veterinary expenses set in, and you are having incontinance issues, etc. Well, it may be too many then.
There are days when I think two would be great. But then I can't imagine life without this one or that one. At -11 degrees F, having just two would be awesome.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC
Hepzibah & Hannah