The only answer I've seen from the ATF is two per form, and that was from multiple people (including a dealer) asking them.

Until somebody does it the old way and reports back that they had the forms sent back asking for more prints, we won't know.

You'd think.

All I have to go by (and the reason I added the note to the sticky) was this text convo I had with local dealer:

Dealer: Did you put on your thread that two sets of finger print cards are needed for each serial number if sending in multiple forms?
Me: I did not. I saw somebody mention the person told them that when they called in and asked. Is it on the form somewhere? Or did you hear it somewhere else?
Dealer: I called the NFA branch and asked this morning for a customer that was buying 2 suppressors.  They said 2 sets per serial number.
Me: I'll add a note. Probably won't know for sure until the first person that just sends in one set gets an error letter a year from now.
Me: Actually, I guess it would be kicked back early in the process, like if you forgot payment?
Dealer: The specialist said the application would be returned without action if missing cards. Basically dissaproved if finger print cards were not there.