How many vials?

Here's something else I had no idea would be an issue- how many vials of the donors good stuff to purchase? Hopefully you only need one and it will work on the first try. But maybe not. And the bank offers a deal if you buy 6 vials. Is that because people need 6???? Then there's the issue of siblings. What if I end up wanting to try to have 2 kids? Would I want the same donor for both? At the moment, I don't care. I'm trying to minimize in my mind the import of the donor, so why would it matter if the two kids are full siblings? I don't know I'll feel in 2 years. I decide that is too much to worry about. I buy 2 vials.(it's $675 a vial, so it's not easy to de die you want to buy excess!). If it doesn't work on the first 2 tries, I'll move on to a new donor.