Causal universes are a bit like integers

Mostly-causal universes with some time-travel elements are more like rational numbers; there's more than we're ever going to use

Rationals and integers are both coutable! This is one of my favorite not-often-taught-in-elementary-schools but easily-explainable-to-elementary-school-students math facts. And they, the rationals, make a pretty tree: http://mathlesstraveled.com/2008/01/07/recounting-the-rationals-part-ii-fractions-grow-on-trees/