@mike23, welcome to StrongFirst.
"1RM" is an abbreviation for "One Repetition Maximum." (We have an article on our forum to explain all commonly-used abbreviations; I see 1RM is not included, but I will fix that after I post this message. The link is
Acronyms (Abbreviations))
Increasing one's 1RM, through training low-rep sets with moderately heavy weight, can yield an improvement, an "increased," in maximum repetitions at bodyweight only, if that's what you're asking. The way to go about it would be to
1. Establish a current maximum rep count at bodyweight only.
2. Establish a current 1RM in the weighted chinup.
3. Embark on a training cycle to improve weighted chinup 1RM.
4. Test weighted 1RM.
5. Go back to a bodyweight-only training cycle.
6. Test max reps at the end of that.
-S-