If you want to add weight to your chin-up don’t worry about doing high reps.
If you were trying to add weight to your bench press, you wouldn’t focus on doing sets greater than 12 reps. Instead you would train your chest with a heavy enough weight that limited your reps into the 3 to 5 range or less.
However, advice often given at the gym is to do more reps. If you can do a set of 12 reps with your bodyweight, you are expected to train for 15. This misconception causes people to miss out on the benefits of weighted chin-ups.
Training heavy will allow you to reach your goal much faster. Heavy weighted chin-up will build your lats and biceps faster than any other exercise. If you can already do 12 bodyweight chin-ups then it is time to start adding weight.
Try keeping the reps low; start with sets of 7 reps and slowly work your way down to set of 3 reps as you continuously add weight.
Start by adding a 2.5 lbs plate and do 3 sets of 7 reps. Each workout add an extra 2.5 lbs. Once failure is reached at any given workout lower the reps to 3 sets of 5 and then 3 sets of 3 reps – repeating the process.
If you use a split training system and train biceps and back on separate days, then do pull-ups on back day and chin-ups on biceps day. In order to progress in this exercise you need to perform chin-ups and/or pull-ups two to three times a week.
The initial weight you are starting with is submaximal considering you can do at least 12 bodyweight chin-ups before starting this program. You should be able to continuously add weight to this exercise for several workouts before dropping the rep range.
I did this exact same program using wide-grip hammer chin-ups and reached a top weight of an additional 100 lbs for singles as a natty. I developed width in my lats and thickness in my biceps. Plus, the strength I gained in my lats and biceps transferred to all other pulling exercises.