I'm weighing in a bit late on this thread ...
The scientist at my fertility clinic told me that they had a 75% thaw success rate. However we recently had a FET and were planning on having 2 embies transferred, leaving 2 in the freezer for a future transfer, or so we thought. Only 1 of my 4 embies survived the thaw
(and resulted in a BFN) ... so for us we only had a 25% thaw rate. Next time we do a stim cycle I'm tempted just to tell them to transfer the lot - I've never fallen pregnant with IVF, stim or FET cycle, so I think my chance of multiples would be low.