For me, I'd say about 20 if we're just counting all three.
(The absolute BEST of which was Trilogy Tuesday. Amazing experience!)
But if we're talking FotR or TTT on their own, that number goes up. Fast.
I don't like to watch RotK too frequently because the movie carries such an emotional whallop for me. It's not that I avoid it, but I don't often seek it out. For two reasons, really. First, some days I just don't feel like crying! And second, I don't ever want to get to the point where the movie stops making me cry!
I don't want to watch it so often that I get so used to it that it no longer works on me the way it should.
If that makes any sense!