The magic number for me is six. Therefore I have six holders for 8x20, 11x14 and 12x20.
12 exposures for any one ULF format is plenty for a cycle of ULF photography. That said I have yet to utilize all of my holders in one day of shooting.However, I firmly believe in having the potential should the opportunity present itself.
The more you shoot the better you see and the more holders you need.
I am with Michael on the number six for ULF. Here is what I have.
7X17 -- Six
12X20 -- Six
20X23 -- Six
The other formt I use is 5X7. For 5X7 I have twenty-six holders.