I have only one crowning file, and it seems to me, that, in a perfect world, you'd need one for each different fret width and profile. If the crowning file is narrower than the fret, the fret width will be reduced to fit the inner concave size of the file. If the crowning file is wider than the actual fret, it will tend to flatten the top and not take much off the sides, and tend to flatten the fret, unless, as I do, use the file (which is wider than the fret) on each side of the fret alternately. I first run the file over the fret with a small amount of pressure to the right, thus bring the left inside of the crowning file in contact with the LHS of the fret, then I do it with pressure to the left, thus doing the other side of the fret. By adjusting the angle the file is held at, you can vary the amount taken off the lower sides and nearer the top of the fret, so you can make some adjustment to the profile of it.
Yeah, that's kinda how I do it too, and I get good consistent results that way. (The method of using it on one side first etc, I mean!)
Although I still honestly prefer just using a 3 corner file for crowning.
I come from a background of jewellery manufacturing, so perhaps I'm simply more confident and comfortable with a "normal" file!! I don't often have to worry about how long it's taking me though!
And yeah, you do get crowning files graded by the width of the frets on which it will be used; medium, jumbo etc
DJ