Before people can request anything, you decide the ceiling.
Per artist. Open an artist and set per accreditation type how many guests they may put on the list. Need to differ per day? Use Set per day and fill in the grid, for instance 4 on Friday and 10 on Saturday.
Per relation. A relation (sponsor, label, agency) gets their own portal with a limit per type per day.
On the public form. Set per type per day how many one person may request at most.
As a hand-out. You can give someone who is already on the list an allowance to invite others themselves (see chapter 9).
Someone can never request more than their allowance. Once it is used up, the option simply disappears from their form, so you don't have to say no afterwards.