What's an appropriate # of airguns to own?

It depends on how many shooting activities you take part in. Someone who hunts, plinks and target shoots needs more.

it also depends on how much time and money you have. If you are spending money on air guns instead of feeding, clothing and educating your kids, it's too many. If you can't walk in your home due to all the air guns, you are a hoarder, not a collector.

I'm in favor of the quality over quantity thing. I usually reach for my favorite gun or favorite 2 or 3 (at the most).