A lot.
If the hair transplant Doctor is bad, even small amounts of harvesting can destroy a donor
Lot's of things can go wrong during a FUE transplant, and that's why most doctors limit FUEs to small sessions. The higher the graft count you go, the risk of you making mistakes and losing hairs grows exponentially.
The more refined your technique and tools are, the more you can extract without making mistakes (or making less mistakes). For example if you damage 10% of the adjacent donor hairs it may not show in a 2k graft procedure, but that kind of % of damage in a 12k graft procedure would make a big dent on the scalp.
If the hair transplant Doctor is bad, even small amounts of harvesting can destroy a donor
Lot's of things can go wrong during a FUE transplant, and that's why most doctors limit FUEs to small sessions. The higher the graft count you go, the risk of you making mistakes and losing hairs grows exponentially.
The more refined your technique and tools are, the more you can extract without making mistakes (or making less mistakes). For example if you damage 10% of the adjacent donor hairs it may not show in a 2k graft procedure, but that kind of % of damage in a 12k graft procedure would make a big dent on the scalp.