Will I get a seat in government dental college mumbai with 500 marks?

You haven't mentioned your Category. Neverthless according to previous years data, if you belong to General Category, the chances for a seat for 500 in Government Dental College might be on border line. You will need atleast 550 and above. But for Resrved Categories, 500 is a good score. Now, as mentioned this is based on previous years data and cut offs change every year. Cut offs change every year based on no.of  Candidates appearing for the exam, no.of Candidates qualified, top NEET score and difficulty of the exam.