Well, we get maybe two or three a year, so that's about 1%. Our ewes are Texel types having Texel and Charollais lambs. We had hardly any prolapses in Swaledales carrying Swale and Mule lambs - I saw over 1000 of these ewes lamb over my time there.
If prolapses are going to happen, it's more common in the last stages of pregnancy, typically in a ewe carrying plenty of condition, tupped to a meaty type of tup and often carrying twins.
Basically the uterus everts / pops out because there isn't room inside! Then, the bladder can't empty, so that begins to fill up, taking up more room... And the everted uterus swells, so it's larger... When you first get to a ewe with a prolapse, as soon as you can safely do it, push the prolapse in enough to get the bladder opening uncovered - and wait. You will get a wet, warm arm and a deep sigh of satisfaction from the ewe! Once she's emptied her bladder, she is much more comfortable and there's more room in there, so you can take your time getting the thing back in and fixed in.
Prolapses don't develop overnight but aren't easy to spot in the field until they are well advanced, because early on they play 'peek-a-boo', so you have to be watching the right ewe for a while to see it. If the girls are indoors you're more likely to spot it early on, when you can sometimes just give her a chance to empty her bladder, pop it back in, and have no further trouble.
Even with a small handful of ewes, if they are meaty types carrying meaty lambs, I would have a bag of sugar and a prolapse spoon on hand - you can't stay with your hand up a ewe's bum all weekend until the shops / vet surgery are open!