we make a hypothesis based on some time in the future. The Bitcoin account is one trillion. Assumption: 256 numbers or combinations of letters are 2 trillion (it can be calculated by mathematical formulas, here we make a hypothesis) Then: any modification of the letters in the private key, it will be the private key of an account .
It also applies to altcoins. But the number of Bitcoin accounts is the largest. It needs to make changes first.
Your calculation are totally wrong,
1. There are 2^256 (115792089237316195423570985008687907853269984665640564039457584007913129639936) possible private key permutation, even though all private key above 0xFFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4140 is invalid on ECDSA
2. There are 2^160 (1461501637330902918203684832716283019655932542976) possible address/account permutation
If you have hard time imagine how big are those number, i suggest you check
https://www.quora.com/Is-256-bit-encryption-breakable?share=1 or
https://www.youtube.com/watch?v=S9JGmA5_unY