Dear student,

First of all you must mention the counseling name.
Assuming that you are asking about neet counseling, then you are not allowed to do registration in second round counseling.
Only candidates registered before 1st round counseling, will be eligible to participate in 2nd round counseling. If you think that - you miss the first round registration and directly register for 2nd round, i.e, after seat allotment for 1st round, then its not possible.

Also you must do the choice filling before 1st round counselling.

Also check out the following link to know about neet faqs :-
https://medicine.careers360.com/articles/neet-counselling-faqs

Also check out the following link to know about neet 2020 counseling :-
https://medicine.careers360.com/articles/neet-counselling