Once a reasonable level of pull-ups have been reached, it makes progression easier if you focus on adding weight instead of reps. Here’s why:

if the trainee weighs 210

210 x 12 = 2510 lbs

The next session he can either:

210 x 13 = 2730 lbs

or add 10 lb

215 x 12 = 2580

While the extra rep is a much bigger gain, but for some people, at some point it's just not sustainable, and then they should focus on adding weight.

Iron Addict