AnimeMania :
What is the name of the standard for the hole in the keycap/post on the key so that you know the keycaps are compatible with your keyboard.
There are several, but the most common by far are the Cherry MX-style post (cross shaped). The switch compatibility is pretty much always clearly stated in any product listing for aftermarket keycaps, just look for it.
AnimeMania :
What do people do when the number of keycaps that are supplied are less than the number of keys on their keyboard?...Is there a standard number of keycaps for a set?
That doesn't happen unless you mistakenly order the wrong set. The 3 most common sets (assuming U.S. layout) include the 100% - 104 key (typical full layout + number pad), the TKL (ten key-less) 87 key, and the 60% - 61 key layouts.
There are quite a few other layouts that require shopping at dedicated sites for compatible caps, but unless you went out of your way to buy such a keyboard, your board should be compatible. If your board happens to have any additional keys not covered by the usual standard counts (e.g. macro keys, multimedia, etc), you'll have to buy those separately.