Meanwhile, there are 45231284858326638837332416019018714005014673546513634524455141852155 possible Bitcoin keypairs.
Do you have mathematic proof for this or are you just guessing that really every point on that curve can be reached?
The group order is known and is equal to FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFE BAAEDCE6 AF48A03B BFD25E8C D0364141 in hex form, however I don't know if that's equal to 45231284858326638837332416019018714005014673546513634524455141852155.
edit: it's actually 115792089237316195423570985008687907852837564279074904382605163141518161494337, which is very close to 2^256.