We currently have four.
I'd say, having experienced life with one, two and three, that three is the perfect number.
The big difference for us in going from three to four was that we no longer walk them singlehanded. This means I do a 2 + 2 walk which takes twice as long. DH is away for a lot of the week so this has really impacted on my time (I work from home). With three, I could do a single walk which helped so much in managing the time element. Having loose lead walking really nailed is a key skill if you have three.
The other thing I'd say is that once you get above two, it's much harder to find places to stay that will take dogs (especially if they are large). A lot of allegedly dog friendly cottages will say things like "up to two well behaved dogs" or make exorbitant charges per dog, which really mount up. You'll also find that whereas it's quite easy to find a neighbour/friend to dog-sit if you need a day out/night away with one or even two dogs, very few people are happy to be invaded by three or four!
We had three for a number of years - as we had two of our own and then fostered, which is a lovely way of doing it. Inevitably, we "failed" and then became three permanently. Possibly because of the utterly lovely dog we took on, we found that to be no bother at all. By that point, we'd already swapped our car for an estate and were used to thinking of dog hair as a condiment.
I'd also reluctantly say that having three or four does take its toll of the house. Even with an allegedly non shedding breed like greyhounds that don't get especially muddy, cleaning is a bit of a Sisyphean task especially if we have a wet winter. I am not remotely houseproud, but it can be very dispiriting when you've just got the house sparkly for visitors, and one of them decides to mysteriously bring feet fulll of oomska in from the garden.