I got my tree frogs for the same reason ^^ (I love my nemo fish as well, but they're not that cuddly.) but one doesn't like me holding her as much as I do, so I leave her alone. of course, you shouldn't handle them too often... (but no one says what "often" means.)

you can keep a single frog without any problem as tom said but they're said to be social, so the more the merrier... and probably you'll get yourself a second one very quickly anyway because they are so cute. (I've had two in the beginning but they turned into three very quickly.)

I have no trouble keeping up humidity. I spray few times a day (by hand) and It's around 50 to 60% during daytime, little more at night... (dunno if it's cause I have plants in it..?) some people might say that's too less, but there are as much opinions about it as there are people... some say you need almost 100% at night, 90% daytime, others say white's don't like too much humidity 50 to 60 is great and our local amph/rept-pet store has slightly less than I do. (the store has to do it more or less correctly, since we have very strong animal rights here. example: you are only allowed live feeding your snake if it doesn't eat frozen mice...)

tank size: no idea. in the US you seem to have waay different opinions about that. here it's stated by law that your tank has to be at least 2 feet high, 1.5 ft wide, 1.5 ft deep if you want to keep on or two white's, but people in forums usually find that way too small. I've got a 70 gallon tank for three and there are lots of people here who'd kill me for doing that - some find that waaaay to small even for two.