Actually, I don't think it's a IATA mandate, just that most airlines are choosing 16 as the maximum coupons per electronic ticket record, and oneworld decided to standardize on this number for their alliance products.

Having said that, with the exception of WN (4 coupons per ETR, which I am sure will increase to 16 once they've fully migrated to Amadeus' Altéa), I can't think of an airline that has a maximum number that's different than 16.

Incidentally, in 2003 I was able to construct a single award with 32 coupons, which, of course, had to be hand written.