i am getting 500 marks in neet 2021 in gen-ews category. which govt. mbbs college can i get in 2nd 3rd or mop up round ?

Hope you are doing well
It is difficult to predict the cut off marks or ranks but according to previous year trends your expected rank would lie between 97259 - 114409 but it may vary every year depending on the following factors
1. Level of question paper
2. Total no of candidates
3. Category of candidate
4. Total no of seats
5. Marks obtained by students
Here is the link for the expected ranks

According to previous years the cutoff marks for government college in up colleges are as follows :
1. Autonomous State Medical College, Basti - 606 marks
2. Government Medical College, Badaun -610 marks
3.   GSVM Medical College, Kanpur - 611 marks
4. Uttar Pradesh University of Medical Sciences, Saifai- 618 marks
5. State Medical College, Shahjahanpur- 608 marks

And 606 iis the last cut off marks for gen ews category and it was for Autonomous State Medical College and Associated Teaching Hospital, Bahraich
So you have less chances to get seat in government mbbs medical colleges.
https://medicine.careers360.com/articles/neet-cutoff-up
You can use college predictor tool to know about the colleges you can get
https://medicine.careers360.com/neet-college-predictor

Good luck
Hope it helps