Just went through this with my FFL I do my transfers through here in Kansas. Prior to this past week, I'd never purchased more than one hand gun within 5 business days from any FFL.
The law reads (and I assume this is federal ATF), within 5 days that the business operates, or in other words, sells guns.
For instance. If the FFL only sold guns on Saturday and Sunday, and you bought a gun on Saturday, another other pistol purchased from that same FFL on the following two Saturday and Sunday's would require a multi buy form to be filled out.
If the FFL, such as mine, regularly does transfers 7 days a week, then any purchased within 5 consecutive days must be on a multi gun form.
I was told by my FFL that he retains a copy, a copy goes to ATF and a copy goes to the local chief of police.
I had purchased one last Thursday and did the transfer. I had two more coming, I had planned to wait past the 5 days to fill out another form, but both came in the past Monday so I picked up both. All 3 pistols will be put on a multi gun purchase form.
I did this in part because 1: I REALLY wanted my guns and 2: picking both up at one time was just a single FFL transfer fee.