In this session, only in CBSE, 99% of students have passed. I don't think you should take any risk by filling less number of colleges. But the same thing if you asked me 2-3 years ago, then you definitely get college in your first 20 filled colleges. This time due to coronavirus, everyone has good marks, and colleges will be allotted to you on the basis of your 12th percentage.
So, a minimum of 25 colleges should be filled, and then wait for the first merit list,