I don't think there is any set number. The coin below was originally listed as an Error (Die Gouge), but it took 7 years before it was changed to a Variety (Die Dent) and listed in the Cherrypickers Guide with only 12 known.
It's up to the catalogers. In general it's a variety (die variety) if it's on the die, like a die gouge, die dent, doubled die, repunched mint mark, or wrong die. And it's an error (minting error) if it's not on the die but a problem in the minting routine. Like a brockage, misaligned die, die break, double struck or rotated. What I'm not real clear on is a die clash and where that falls. I know left over polishing lines from removing a clash would be considered a variety because you can use it to identify a die or die pair, like in Morgan or peace dollar VAMs. As far as die gouge or die dent, there's always been some confusion in the hobby correctly identifying one from the other. A die gouge is usually more like a really deep die scratch or a "cut" that's incused in the die. A die dent is more like a indention incused in the die. When it's a linear die dent this is where the confusion of whether it's a gouge or a dent appears to happen most of the time.
All I know is it's like a bump of extra metal in the hair like a chip out the middle of the die not big but noticeable, almost like extra hair
The answer is "one". A die variety is defined by its etiology (manner of formation), not its abundance.
