I was wondering, how many fish you can keep in one tank in the Windows version?

You can keep 21 fish in each tank. From what I understand, they added room for one more when they came out with the windows version to make room for the seahorse.

One more question. If it is in screensaver mode, do your fish still age?

Yes, your fish will still age if you run it in screensaver mode. I find it very helpful when I have to do something else that doesn't involve my computer, but I also want to watch my fish.