An All India Rank of 20,000 means around 505 marks in NEET UG Entrance Examination. Assuming you belong to general/unreserved category, it is very difficult (or you may say impossible) to get a government medical seat via 15 percent All India Quota.
However via 85 percent State Quota, you may get a seat but that depends on the state where you reside. Example, in Kerala the closing rank is within 5000 for general category students. But in Tamil Nadu the closing rank is 70,000.
Hence I would say you to comment below your state and category so that I can say you what marks exactly you need to get a seat in a government medical college.

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