Number of spawns under a given Vampire?

[Tetujin]

I'm writting up a character for an epic one(or two)-shot I'll be playing in. I'm playing a vampire that with a feat can control 64HD of spawned vampires, and just to review, a vampire with 5 or more HD can take spawn of thier own. Now if a vampire only takes spawn of 5+ HD that means there are a limitless number of vampires under a given vampire, but obviously there aren't a limitless number of creatures to become spawn of 5+HD, so what I need is a way to calculate the number of vampires under the control of a given vampire that includes some variance based on the avalbility of creatures of more then and less then 5HD.