I have good intentions for setting limits, but they aren't going so well!
I live in a very small apartment and had originally placed pretty strict limits on how big I would let my collection get. Also, I have to keep my ponies secured in some sort of container for their own safety. My naughty kitty thinks everything is his and forbidden items are most definitely the best toys. I can just imagine coming home to him chewing Mimic's horn off.
My original plan was to only collect ponies that I totally loved and that my collection must be able to fit in the nice, clear storage bin I had for them at the time, which was probably about 12 ponies maximum. Then, after a few extras joined the herd, I told myself that it was okay as long as they didn't outgrow the three to four drawers that I had allotted for them in a rolling cart. I've REALLY, REALLY tried to adhere to that rule, but those drawers are getting a little crowded and I know there are plenty of ponies that I'd still like to have someday.
I was randomly looking at some storage bins/containers at Bed, Bath, & Beyond today and caught myself thinking, "I wonder if these are tall enough to accommodation unicorn horns...."
I didn't buy anything, but I know that the temptation is there to continue to expand.