How many photos can iPad handle?
Assuming that each picture you shoot is 1 MB, 20000 pictures will take up 20000 MB which is approximately 20 GB. Now if you put these on your iPad 2, you have around 40 GB to work with keeping the reserved space in mind, which is okay. Looking at that information, you can upload 40000 more if you need. An alternative is to upload them all on a photo-hosting website to save all the 20 GB if you don't look at them often.

Responding to your second query, yes: it is quite responsive. My uncle has 8436 pictures and 129 videos on his iPad 2 and it is working perfectly.