Maximum number of pages on iPad, and maximum apps in a folder?
There is a maximum of 11 pages of apps on your iPad.  Those that don't show up can be launched by swiping all the way left to the iPad spotlight, and searching for the app title.  As for maximum number of apps in a folder, I was able to fit 20 (as you mentioned). 

I was not able to find any official documentation regarding this matter.