I think I'm gonna get 100 marks in kcet, what rank would I get?
You might get a rank around 1,000. Exact rank cannot be precisely predicted. Your KCET score is 100 marks so your expected rank will be in range of 1000. Although it is expected rank, actual rank may differ from it.
It depends on the cutoff.
You can visit the below link, to know more info about your rank.
https://engineering.careers360.com/articles/kcet-marks-vs-rank
All the best!