5000 repetitions for conditioning

Surely you've heard this?  "Everyone" knows research shows that it
takes 300-500 repetitions to develop a motor pattern. Conversely it
takes about 3000-5000 repetitions to erase a motor pattern. Therefore
it is important that movements be done correctly to avoid mechanical
flaws that could result in poor performance and injury.

OK, which study is this from? How was this established, and who
performed the study?