There is no law against buying 100 handguns at a time. The ATF rule that FFL's must follow is that if a purchaser buys more than 1 (one) handgun within seven days, then the FFL must fill out a "multiple purchase" form and mail it to the local/regional ATF office. Obviously, if you buy two handguns at the same time, then the FFL will be obligated to send in the form. The extra paperwork is done on the dealer's end--the dealer may not even tell you he is filling it out!
A couple of notes:
1. This ONLY applies to handguns.
2. This can be easily gotten around by simply buying at multiple FFLs, if you do not want the ATF to have a direct heads-up of what you are buying. I would think they MUST be databasing such materials--because of the temptation to do so and because they can....
3. If you only want to deal with one FFL, and he has two or more handguns in stock that you want, then simply buy one right now and put the other "on layaway" with a deposit and buy that other one 8 days later on a different 4473 (yellow sheet).
4. If you do not care what the ATF thinks, then buy as many handguns as you want, all at once. There is no law against it.
(edited to correct spelling)
(edited AGAIN to correct grammar)