The number is always changing, but at the current count, I think it's about 40, plus all 12 of the Tama-Go figurines released to date, various non-Tamagotchi pets, and accessories. The collection picture in my signature is the most recent one, but it's not updated. I'll probably update the picture once I get the chance during my winter vacation.
I know a handful of members who have hundreds and hundreds of them. As far as caring for all of them goes, you don't. I think people who do a whole ton at a time rotate through their collection or some similar system. Personally, I can't care for more than 5 or 6 at a time. I think that's my max and I'd only be able to do that if I were on vacation. If I'm in school, 3 or 4 is my max, with the majority of them being ones that I know how to care for backward and forward, so it doesn't feel as overwhelming.
I had a little over 20 but I'm selling half of them to make a little of my money back. I'm trying to keep just two of each version. Just in case on craps out, I'll have another in its place.
I used to have a whole bunch of P2s and three Tamagotchi Angels, but that was a really long time ago. I usually run two at a time, rotating them. During summer vacation though, I run all of them at once! The challenge is fun!
I have 21. I just hit 21 with my new V2, but quite a lot (less than half though!) are broken and even more are Tama-Gos or other versions I don't like. I'm only allowed to run two at a time, and it's a REALLY hard decision, but I'm currently running my 4th Generation V5 and 1st Generation V2 (first character on the V2!).