I will take all the twins I can get, one year we had 6 in the first 12 cows and none after that. In general, I hope we get 10 percent plus, usually it is more like 5 to 7%. This year we are sitting at 14 sets in 135 cows calved. 13 of those sets had 2 live calves at birth, all but 2 have calved on their own, some on their 2nd and 3rd sets. I had one calf that was backwards born dead, had to pull another set that was tangled. Unfortunately 1 calf in 1 set was layed on by its mother an hour after birth and another calf in another set was stepped on in the barn. I now have 11 extra calves that I can use for grafting, or with a little extra groceries during the summer for both mom and babies, I have 1000 plus lbs of calf from a cow that would normally give me about 650 lbs at weaning. I keep every female from a f/f set for replacements in the hopes of twinning genetics carrying on in the offspring. I find that we have more twins if cows are fed well during their flushing process at rebreeding. In general, we get them during the 1st cycle of calving season, though that theory has not been completely true some years either. I do not buy calves or bring in calves for grafting on cows that have lost calves as we keep a closed herd for disease purposes. we had a wreck with scours one year bringing home calves from cows I had on feed in a feedlot and lost 25% of our calves from scours we could do nothing about. Twins help make a cow pay that would otherwise go dry and into the sell pen.
A few years ago I read about a university in the states doing research on the inheritability of twins and I believe they were at something like 25% twin rate.