The pregnancy rate in our clinic, when we transfer 4 frozen-thawed embryos, is 45% per cycle. The reason for the high pregnancy rate is twofold. Firstly, these are often excellent quality embryos (it is usually because the parents have achieved a pregnancy with the fresh embryos, that they are willing to donate these frozen embryos to another couple).
Also, since we prepare the endometrium using hormones, the uterine receptivity to these embryos is usually very good.