We currently have four greyhounds - this was not planned, and we previously thought we would stop at three. I can say quite categorically that although I am completely and utterly soppy about our four, I NEVER want to have four dogs again.
The biggest thing is the time taken - when we had three, I could walk them singlehandedly, when DH is away in the week. With four, I do two, then home then the next two. Along with feeding and care, it quite often feels that a substantial chunk of my day is simply spent on dog care. 10 minutes clicker training for 1 becomes 40 minutes for four. If you are a conscientous dog owner who wants to give individual training and attention to each dog, you need to sit down and think about the time taken for this.
You also need to consider transport (especially if a big breed) and don't underestimate the extra costs of feeding, vet care, kennels, insurance etc. We are lucky in that two of ours are fosters so we get help. We get through industrial quantities of feed - you need to be organised in ordering and stocking. You need room for all the coats, leads and paraphernalia they attract.
Four large dogs also has an impact on the house. It always makes me laugh when I read on MN about how childless people live in these beautiful, clean houses with white sofas and priceless objets dotted about. I feel like I live with four hairy toddlers who spend their time finding new and inventive ways to make a mess (and are very good at it). Our sofas are falling apart, our bed is full of dog hair, and our walls are constantly splashed with mud and oomska. Four just are much, much messier than two. Some days I just want to cry, as no matter how much cleaning I do, it just doesn't make an impact (especially with the weather we've been having this year). If you are fussy about your car, also - that has a similar impact. Ours is a smelly, hairy, muddy experiment in new ecosystem development. Thankfully, we only mostly give lifts to other dog nuts. Any normal person would probably scream and require a biohazard suit.
With two, you are likely to still be able to find a relative who might take them for a weekend - with four - forget it. With two, you will be able to find self catering accommodation very easily. With four, you have to search high and low. We have found a wonderful place in Pembrokeshire that has no upper limit on the number of dogs brought, but these are few and far between and you pay handsomely for the privilege.
If I won the Lottery tomorrow, these would all be surmountable problems, as I'd have a gorgeous baronial pile, a fleet of cleaners, and a converted van just for hound transport. But back in the real world - there are impacts and I'm trying to give you the non romantic side of it.
The Kodak moments are there of course, as you'd expect when we all pile on the bed together for a cuddle or are playing on the beach, but don't underestimate teh time, cost and impact they will have.
For me personally, I think three is the perfect number - enough to be a gang, but still walkable singlehanded.