Like others have said, sooo many variables... of which who's doing flushing, who's doing the implanting, donor nutrition, recep nutrition, weather, genetics (some cows/bulls work better than others), etc.
I have had great flushes result in 0 pregnancies (0 for 32 from 3 flushes) and I have had small flushes result 5 out of 6 pregnancies.
The recep herd I use typically run 70-73% conception on 200+ embryos year in and year out. Alll frozen.
If you run into poor results you can general remove most of the variables and pin point where the change needs to be made. From my experience, ET is a technology that is constantly being tweeked, with results that general will be "Wish I had gotten one more..."!
Also, keep in mind, this is not a "get rich quick" fix, or a "gonna buy the perfect animal" situation. More have lost it all, than ever struck it rich!