As far as I know, it is exactly 50% of keys. For any public key you can negate it and switch it from 02 to 03 or from 03 to 02. It is totally symmetric, I cannot see a single example where it would be ambiguous or where more than a pair of keys could be produced in that way. You can see it simply by using G and -G and incrementing or decrementing points. You would see a pairs of private keys: (1;-1);(2;-2);(3;-3);... When dealing with private keys, you can simply subtract your private key from the maximum and get your negated private key. For public keys, you can switch between 02 and 03 prefix. That's all.
privateKey="FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD036413B", publicKey="02FFF97BD5755EEEA420453A14355235D382F6472F8568A18B2F057A1460297556"
privateKey="FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD036413C", publicKey="032F8BDE4D1A07209355B4A7250A5C5128E88B84BDDC619AB7CBA8D569B240EFE4"
privateKey="FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD036413D", publicKey="03E493DBF1C10D80F3581E4904930B1404CC6C13900EE0758474FA94ABE8C4CD13"
privateKey="FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD036413E", publicKey="03F9308A019258C31049344F85F89D5229B531C845836F99B08601F113BCE036F9"
privateKey="FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD036413F", publicKey="03C6047F9441ED7D6D3045406E95C07CD85C778E4B8CEF3CA7ABAC09B95C709EE5"
privateKey="FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364140", publicKey="0379BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798"
privateKey="0000000000000000000000000000000000000000000000000000000000000001", publicKey="0279BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798"
privateKey="0000000000000000000000000000000000000000000000000000000000000002", publicKey="02C6047F9441ED7D6D3045406E95C07CD85C778E4B8CEF3CA7ABAC09B95C709EE5"
privateKey="0000000000000000000000000000000000000000000000000000000000000003", publicKey="02F9308A019258C31049344F85F89D5229B531C845836F99B08601F113BCE036F9"
privateKey="0000000000000000000000000000000000000000000000000000000000000004", publicKey="02E493DBF1C10D80F3581E4904930B1404CC6C13900EE0758474FA94ABE8C4CD13"
privateKey="0000000000000000000000000000000000000000000000000000000000000005", publicKey="022F8BDE4D1A07209355B4A7250A5C5128E88B84BDDC619AB7CBA8D569B240EFE4"
privateKey="0000000000000000000000000000000000000000000000000000000000000006", publicKey="03FFF97BD5755EEEA420453A14355235D382F6472F8568A18B2F057A1460297556"
Yes, its 50/50 ofc. each X have 2 Ys. The point is to get deccion by NN to get correct first half or second half. 02,03 its compressed, uncomopressed form have bytes too.