H = ripemd160(sha256(ECMultiply(A,G)))
A - 32-byte number,
H - 20-byte number,
G - generator point,
ECMultiply - elliptic curve point multiplication.
Is it true that there is many A that have the same H?
Yes, it is possible but the possibility is too low.
SHA256 hashes are almost unique. I don't think that anyone has found a SHA256.