Technically, you are a film composer the moment you write original music for a film, I suppose. But one should draw a distinction between working on a single project and a wider profession as such. But where does the line go between sporadic and profession? You and I both started threads on composers who’ve done just one or a handful of scores, Gerate, and I’m none the wiser.
I suppose it will be an entirely subjective interpretation. I could put out a random number, like 5 media projects, in order to qualify, but that would still exclude a great many composers who have written great music for a handful of films. It’s very tricky.