cats are independent animals. if they think there are too many of them some of them leave. unfortunately even if you have 1 only he/she might leave. where I lived before I had cats who gave birth in my (huge) garage. there were times I had 10+ cats till they found people who fed them tastier...
I had the same problem and never adopted a cat. someone would think God created cats to live in the house!
I started to feed cats that showed up at my property and luckily one gave birth here. I took care of the kittens and 2 of them stayed here. I still feed every single cat that comes here...