We've currently got 4. Things to consider as the number of dogs rises:-
Your car - after our second greyhound arrived, we moved up to an estate car and now we are pretty much stuck with that or a van. Obviously this would depend on the breed you choose.
House - I don't think I was really prepared for how much extra mess each dog adds to the general levels of cleaning in terms of mud, hair, oomska, occasional accident etc. We have a short haired breed but I sometimes feel that cleaning the house is a never ending and thankless task. One dog was very easy to clean round, but I did notice a big jump when we moved up to two. At this time of year, I seem to be in an endless cycle of washing muddy dog coats, our dog walking jeans, throws, dog beds etc.
Financial - extra costs of food, vets, kennels, dogwalkers and insurance plus additional coats, collars etc.
Training - I regard this as essential and ongoing - you have to devote more time to training including separate sessions for each dog.
People you can use as dogsitters - it's incredibly easy to find a person who'll dogsit one eg. next door neighbour, parent, friend. Much less so for two, and for four, only if like us you have friends who are also multi-dog obsessives.
Holiday accommodation - becomes steadily harder to find as the number of dogs increases.
Having said all that, I am completely smitten with ours and love being a multi dog household - it's just so much fun. And it's endlessly fascinating watching the dynamics of the group, the relationships and how they help each other get into mischief etc.