Sorry I was away for a few days: I came back and noticed that stuff I had written was being quoted and all heck broke loose...
Anyway, if Ott doesn't have the answer to the number of ex cathedra pronouncements, I'm sure there is no set definition.
From a practical standpoint, I can say that I never encountered this as an issue until I saw posters here point out that there was no such list. Armed with that question, it does seem strange.
I was taught the following were ex cathedra during my catechesis: the Immaculate Conception, the Assumption, and Humane Vitae on artificial birth control. I've also been taught later that Ordination Sacerdotales should be included as well.