If I get 91 marks in PCMB in kcet exam 2021 ,then what rank i got,

As per your marks it's also depends on how many candidate apply.

The score 91 out of 180 it's good marks

The rank will be between 3800 to 4500

For more information you can visit here

https://www.careers360.com/exams/kcet

And also you can see marks vs rank from below link
https://engineering.careers360.com/articles/kcet-marks-vs-rank

You can also predict your college as per your rank from this link

https://www.careers360.com/kcet-college-predictor

Thank you. I hope this answer helps you.