I've been singing versions of some of the Child ballads for years but, despite being enough of an anorak to keep a record of what I sing and where, I've never bothered to notice how many verses – like others here it just takes as many verses as is needed to tell the story. I used to go to the ballad sessions at Sidmouth festival (which I'd recommend to anyone interested in traditional narrative ballads), but I couldn't tell you how long each ballad was. Most of the singers were good enough that I'd lose myself in the stories after a couple of verses – no counting!
(A while ago I felt the urge to make up a few songs based on Welsh folklore (such as the birth of Pryderi; Pwyll's meeting with Arawn; the rescue of Mabon etc.) The only way to tell the story was via a fair number of verses although, again, I didn't bother about the actual number (a quick check now shows between 10 and 19 verses was the norm). Of course, there's really no-where to sing these songs "out" as most singarounds these days focus on chorus/refrain songs, "clubs" tend towards more usual singer songwriter introspection and, of course, if you go to a place where ballads are welcome it would be a crime not to try to keep some of the traditional ones alive. Still it was fun trying to create ballads in a traditional style that just gave the bare narrative of the story.)