There is no way of pre-determining how a donor will respond to super-ovulation or how many useable embryos she will produce. To improve the odds of a successful flush a donor that has calved 50-90 days and has a good reproductive history and relative youth will be your best bet. Donors should be on an increasing plane of nutrition with no specific nutritional deficiencies. Cows with condition scores from 2.5-3.5 (moderate flesh) out of 5 are preferred. Overfat cows generally create problems. Virgin heifers and cows that "come up open" are less predictable. The most often asked question is "how many embryos can I expect from my flush?" My pat answer is "from zero to eighty". The average is six to eight, but averages are made up of any combination of numbers, including zeros. Averages are also meaningful when there are at least 50 flushes included.
ET should be limited those cows with superior genetic merit. Just because the letters "ET" are in an animals registered name, it doesn't make them genetically better or worth more money. ET is a great way to disseminate genetics of the "elite" cows. It will also disseminate undesirable genetics, so make sure your donor is free of known genetic defects.
When purchasing a proven donor cow, check her history of embryo production and calving history.