My clinic also thawed one at a time.

I had to sign a form asking if I wanted to just thaw one and see what happened or if i wanted to thaw one and if it didn't survive then try another until they got one that thawed correctly. The nurse told me that they had had issues with someone not understanding that they may have to thaw more than one before they got one that survived but they would always only thaw them one at a time. Apparently they can tell pretty quickly if there is nothing going on with one.