Hey !
NEET mopup round is conducted after the two rounds of counselling for those collehes where some seats are still vacant. You need to apply for this MOP UP ROUND COUNSELLING by filling a separate form only when you couldn't get a seat in first and second round. The dates for registration for mop up round are not out yet. You just need to apply for this round by filling the online application form and uploading the required documents.

To know the possibilities of colleges where you can get a seat , use NEET College predictor tool;

https://medicine.careers360.com/neet-college-predictor?icn=QnA&ici=qna_answer