I had two, sort of. The first was on day 3 of my cycle, and it was just to make sure that I was actually having a period since I was coming directly off a failed fresh cycle. The second, on day 10, was to check the lining, do the mock transfer, and since they were in there anyway they do a little endo scratch by wiggling the trail transfer catheter around a bit.
I had the same with the fresh donor cycle - one on day 3, the other just a few days before transfer. Since my ovaries weren't being stimulated, I didn't have to go through as many as with my own cycle.
I'm not certain how many places skip the day 3 check since all it should show is a very thin lining that you might still be shedding; I wouldn't be surprised if quite a few did. The later check is the important one, and mine showed an intermediate striation patter, not the triple layer they hope for, so don't worry about that part if they mention it.