I think 15 teams will use 6 OTA days this season. A few use less than 6. Only 7 teams use all 10 days.
OTA’s are mostly meant to get guys acclimated, rookies introduced to the team’s process, and some classroom time. No hitting, no pads. There is some walk through.
And just because most coaches choose to use 6 of 10 doesn’t mean players aren’t doing anything. Especially the rookies and new teammates. A lot of coaches choose to take 3-4 of the OTA days and spend them strictly with rookies. This gives the coaches more time with the younger/new guys.
I don’t think there is any evidence that supports 4 or 6 or 8 or 10 OTA’s are the best way to do things.