Context is indeed the key! I am not "demanding" an optimal number, I am asking by what process and using what rationale (or what kind of rationale) such a number might be determined.
@PureX has at least made a suggestion - that the number be determined by polling the population on the question of how much is needed to live a safe and comfortable life and then doubling it...and after some pages of discussion, supporters of the idea do seem to be settling somewhere in the tens of millions (compared to the variance of almost three orders of magnitude that were being bandied about earlier). I think it is important that there would be at least some idea of what the limit might be and how it might be determined BEFORE it was imposed, don't you? Not least because you would, presumably, want to know (at least approximately) how much revenue the tax might raise and what kind of effect imposing it might have on the economy...how many investors might have their investments stripped, how many of the poor might actually benefit, by what redistributive process(es) and by how much.
The problem with "in principle" impositions is they sometimes fail to anticipate the (sometimes catastrophic) economic consequences..."turn gold into bread" sounded good in principle but that's not exactly how the Bolshevik redistribution of wealth worked out, you may recall.