I looked around a bit and couldn’t find a good straight-up estimate. I may be able to give you a decent estimate for UC patients alone, though.
Given that between 35-100 people per 100,000 in the US have or will have ulcerative colitis in their lifetime (estimated around 40 per 100,000 diagnosed, currently), and the current population of the US is 311,591,917, that gives us something like 125,000 UC patients. About 1 in 3 (~41,500) require surgical intervention for their condition, and about 80% (~33,200) have a K-pouch (or ileal pouch–anal anastomosis) for some length of time.
For ulcerative colitis, the Koch pouch is the preferred surgical treatment, and is generally effective, though there are sometimes infections and complications along the way. This isn’t including the Crohn’s disease and other bowel disease patients who have K-pouches, but as UC patients are the vast majority of the candidates for the operation, I figure they probably constitute the majority of those currently living with the procedure.
[tl;dr: probably around 30,000]