The coupon collector's problem is a well-known example in probability.
Searching the site for
coupon is:question suggests we have about a dozen posts on it, but that searching is complicated because we have e.g. some business/marketing questions where "coupons" turn up in the retailing context.
It strikes me that
[coupon-collectors-problem] (that looks grammatically shoddy but I don't think you can have possessive apostrophe in a tag name?) or maybe just
[coupon-problem] might justabout be justified to tie these related questions together. Or is this too few to be worth doing?
(When we had fewer it would have been much easier to pick them out from search results. My personal inclination is we now have enough, and searching is somewhat muddied by other uses of "coupon", that it's justified to start a new tag.)