User blog comment:Malthael Archangel of Death/"The end" timeline/@comment-176.221.123.146-20180627184520/@comment-2605:A000:122A:408A:2CEB:26E1:2592:196E-20180807143048

Like time travel, alternate universes are handled differently in different shows and stories. In AU, Sam and Dean were never born. So that reality would have paralleled Earth´s until Mary´s decision to not accept Azazel´s deal. If you believe in free will, then Mary´s choice altered the foreseen reality enough to create a different one.

That implies that, yes, there could be infinite alternate realities, but not actual worlds in the sense that you could travel to them in a space ship.