How many of each kind of animal can you have on your home street?
I've been testing it but it's difficult to put together the results because sometimes they'll be happy with 8 and other times they'll get all grumpy with 6.
If you have any idea what the limit might be, post it here :D
Same as Ygasuasu said, but add 3 to the number of chickens i think. I haven't experimented with butterflies yet and if/how they offset pigs. Other people have affirmed me though that butterflies are the same case as chickens.
I've found that if you leave for a while and then come back they'll be happy again. For instance, I have 7 butterflies and 5 chickens on my home street, all of which are happy but before there were one or two of each that were not.