Plenty, most of the times with 1 hand card in holdem. In Omaha, obviously more, even I play significantly less. Quite strange, that in Holdem, I remember only one backdoor, 2 cards in hand, but in Omaha, it is - can't say common -, but happens more often I thought, I have counted the first 6-7, after I lost count, but it is very strange. Especially, in Holdem I can say, it is 50-50 for me and the opponents, but in Omaha it is only around 20-25% for me... also strange, how come, that it happens 5-10 times more often... it is just an estimate, but significantly more often...