No minimum nor maximum. Generally figured, the more grooves, the more "edges/corners" to control the projectile.
For best results though, it is generally accepted to have wider grooves than lands. I've had a couple that were opposite and they still shot well,
but not well enough for me and they were replaced.
The 2-groove barrels were an English Item, normally designed for a "belted ball" or with 4 grooves like the Jacob Side by Side "military" rifle, for a bullet cast with 'wings' to fit the grooves.
It used solid, as well as explosive projectiles.
Here's the muzzle of Taylor's Joseph Lang rifle. 10 grooves, 10 lands grooves 1 3/4 times land width.
It shoots very nicely, 2 1/2" for 5 shots at 100 meters, first time at the range.
This is my .69's bore. it also shoots well, slightly better than the rifle above and has 8 lands and grooves, grooves slightly wider, about 25% than the lands.
This is a Dickert muzzle with narrow grooves, wide lands, generally harder loading with tight patched balls.
