Four is my current limit. Why? because that is how many I have now. My hubby is very sweet and told me that he would never put a limit on how many birds we get, not that he'll be happy about getting #5, but also wont throw a tantrum. He wants me to decide when enough is enough, they are MY birds and I will be the one to take care of them.
My limit is four now because I honestly cannot handle anymore. We have Scarlet who requires more out of me taming wise than Jake. But, Jake is a velcro bird and so if I spend more time with Scarlet than him chaos ensues. Then there are the fosters. They are funny birds, but they are literally wild. I have taken on the task of taming them so I can get them adopted quickly. I only have so much time in my day and only so much space. I think it would be unfair to any new bird, as well as my current birds to take on anymore birds. Like Rue said, the urge for "just 1 more" quickly fades.
We live in a condo with limited space. Scarlet's and foster's cages are 5 ft tall and 3.5 feet wide, which are two large cages to keep in our only extra bedroom. This also means that I have no proper quarantine space, unless the bird is no larger than a small conure or cockatiel and so it can be kept in our bedroom.
Of course the rescue just took in a gold-capped conure, which is VERY tempting

.
We (hopefully) will be moving to a larger home in 2 years. When that happens I will consider adopting one more, but for now, I am very happy with my current mess makers.
Many have forgotten this truth, but you must not forget it.
You remain responsible, forever, for what you have tamed.
-Antoine de Saint Exupery