I'd say half a dozen or so in 40 years of flying. The worst was two on the same flight. Brutal winter conditions going into Sault Ste. Marie on a Nordair 737. The pilot made two approaches and on the third attempt put the plane down in the hardest landing I've ever experienced. Overheads flew open, O2 masks dropped, a food cart came loose. You know it's a rough landing when the cabin crew look shaken.