If you type in "Presidential map 2020" google shows a map with counties. I used that commie map, should be worst case scenario. Some of the counties were hard to count, very small or oddly shaped with very close borders. I'd say give or take a few, so I'm guessing 524 or 525 is correct.

(Alabama 13), (Alaska 8), (Arizona 5), (Arkansas 8), (California 33), (Colorado 23)(Connecticut 6), (Delaware 2), (Florida 12), (Georgia 31), (Hawaii 5), (Idaho 3)(Illinois 14), (Indiana 5), (Iowa 6), (Kansas 5), (Kentucky 2), (Louisiana 10) (Maine 8), (Maryland 12), (Massachusetts 14), (Michigan 11), (Minnesota 13) (Mississippi 28), (Missouri 3), (Montana 7), (Nebraska 2), (Nevada 2) (New Hampshire 8), (New Jersey 15), (New Mexico 12), (New York 16) (North Carolina 24), (North Dakota 2), (Ohio 7), (Oklahoma 0), (Oregon 10) (Pennsylvania 12), (Rhode Island 5), (South Carolina 13), (South Dakota 6) (Tennessee 3), (Texas 22), (Utah 3), (Vermont 13), (Virginia 39), (Washington 13) (West Virginia 0) , (Wisconsin 14), (Wyoming 2)

Biden: ~530 Total: 3,143 Percentage: ~16.9%

EDIT: Feel free to reply with correct numbers. It seems 530 is off by roughly 6 counties, count should be 524 I think.