If an effect would create one or more tokens under your control, it creates twice that many of those tokens instead.
All of the tokens enter the battlefield simultaneously. They’ll have the same name, color, type and subtype, abilities, power, toughness, and so on.
If the effect is putting more than one kind of token onto the battlefield, it’ll put twice as many of each kind onto the battlefield. For example, if you cast Bestial Menace while controlling Parallel Lives, you’ll put two Snake tokens, two Wolf tokens, and two Elephant tokens onto the battlefield.
If you control two Parallel Lives, then the number of tokens created is four times the original number. If you control three, then the number of tokens created is eight times the original number, and so on.
If the effect creating the tokens instructs you to do something with those tokens at a later time, like exiling them at the end of combat, you’ll do that for all the tokens.