I think timing matters as much as numbers. I have three and that's worked out well. If having the 3rd hadn't been such a production, we might even have decided to go for a 4th. But if having the 3rd hadn't been such a production - she came along almost 4 years later than planned - three would have been a real handful! It was the gap between them that made it as easy as it was, easy enough that we thought about having one more so DD10 would have a close-in-age sibling.
ETA: That's for kids I've raised since birth. We currently have 5 kids in our home, the other two being kids we invited into our home when their family situations became untenable, and that's probably my limit. My house just isn't big enough for any more people! But they were both teens when they came into our home, and I'd take teens over toddlers any day so my "limit" for birthing children and raising them from the start has always been lower than my tolerance for becoming a second mother to the older kids in our lives who need a bit of extra support.