Max Tegmark covers this in Our Mathematical Universe. It's a question of definitions. He makes the argument that defining "universe" to mean "our visible unive

Max Tegmark covers this in Our Mathematical Universe. It's a question of definitions. He makes the argument that defining "universe" to mean "our visible universe", that is, a radius of around 140B ly, is the most useful; if you assume infinite space and infinite matter (which seem to follow from the theory of inflation, which has provided explanations and predictions elsewhere), there are likely infinitely many universes all around following this definition (for other visibility scopes).