Hypothetically speaking, and I'm putting an emphasis on the "hypothetical" part, you could have two individuals starting a new population of arbitrary size, wit

Hypothetically speaking, and I'm putting an emphasis on the "hypothetical" part, you could have two individuals starting a new population of arbitrary size, without ill-effects of inbreeding. There is only one requirement. And one caveat. The requirement is that both individuals need to be free of genetic defects in their alleles, so that no matter how many generations down you go you never end up with two defective alleles in the same descendant. That is impossible in practice. Eventually you would get some diversity from random mutation and at that point you would be back to "normal" state of affairs. The caveat is disease resistance. All of their descendants would have the same immune system. If one gets sick, they all do. Any infection will spread like wildfire, if it spreads. And nothing short of total quarantine would stop it.