What's the minimum thread engagement for mounting a red dot on a pistol? How many threads?

Not light reading: on thread engagement

Sometimes, folks are surprised at how few turns 6-32 x 3/8 screws get on Glock MOS RMR plates. As long as it meets minimum thread engagement, screws will do their job, anything beyond is diminished, or mostly no return for more thread engagement.

Glock's OEM plates and our plates are of the same thickness, use the same 6-32 x 3/8 screws, and have the same thread engagement.  If thread engagement were a deciding factor (it isn't), our plates would have the same performance as the OEM plates.

I have yet to see a factory plate that doesn't offer at least minimum thread engagement, as long as it meets that, thread engagement ceases to be a factor.