Hi
I am currently on a FET cycle. I am having my transfer tomorrow. Ours were defrosted on Wednesday and needed to be grown to blastocyst. I got a call from the embryologist at 9am to ask how many I wanted thawed etc. I also had 10. I chose to have all 10 thawed and grown. He said he would call back in an hour to let me know how many survived. 9 of those 10 made it. So I am guessing it only takes about an hour until they know. If you decided to thaw 1 and it didn't make it I "think" they can thaw more. I'm not sure how that would work. I'm sure you can call the clinic and have more thawed out if you want. I'm not sure about the paperwork. All I had to do was sign one that gave them permission to use our embryos, it said nothing about the numbers. But when I have gone in for my blood tests the nurses always ask how many I would like thawed. I would call the clinic and ask them, maybe speak to the embryologist if you can and they can advise you what to do.
I hope that helps you a little, and good luck with your FET