As a joke, I'm imagining someone being confused about the US motto, and thinking it was "e unum pluribus", which hypothetically might mean "out of the one, many" or similar. But I bet that isn't grammatical. So (just as info) what would be grammatical? And could the confused version even sorta, kinda, have the hypothetical meaning?
E unum pluribus has just the same meaning as the original (though you might better use the ex form of the preposition when it precedes a vowel).
The reverse, 'many out of one', would merely require the cases to be reversed, giving ex uno plures.
I like Tom Cotton's suggestion, and I will offer a variant of it. In the original motto e pluribus unum the "one" is neuter. By analogy, I prefer to make the "many" of the new version neuter:
Ex uno plura.
From one, many.
The reason why the translation suggested in the question does not quite work is Latin case inflection. For example, the preposition e(x) requires ablative.
In this neuter reading "many" means "many things" without further specification. If you want it to refer to "many people", "many peoples", or something in that direction, then Tom's suggestion is better.