getting ready for FET

over 8years ttc • . 4failed ICSI IVF already and nothing left to transfer. 6 failed IUI's and lots of meds. Adoption home study completed and approved. We have a little boy!!!!
I'm almost at my next transfer time! I have 4 frozen embryos left and hope to transfer early next week. I went in on Monday and my lining was 12!! Amazing. We re doing a non ovulatory cycle using day5 ICSI IVF. Check up tomorrow and hopefully find out date if transfer. Do they "unfreeze" all of them first and then pick? Also, can u loose some during the unfreeze part?
Update: had my transfer today. My lining was at 14mm!!! Best I've ever had. The transfer went slow and they had difficulties again due to my lovely uterus and cyst and scarring. But we made it through. 2ww begins!