I've been loosely keeping track of it for years, and it has very rarely ever gone over 5 crews per plane. I do this by simply dividing the active roster(s) + people filling a seat on IOE, by the fleet size.

I've seen it as low as 3.5 during very painful periods. The only time I've ever really seen it higher than 5 is during the process of parking a fleet type. The normal average sustained number is usually somewhere between 4 and 5. Usually closer to 4.

I'm hoping this goes higher under 117. But I doubt greater than 5 crews/plane will ever be continuously sustained.