There are many questions in the universe that, not even we, planet overseers, are able to answer. Mysteries that have blinded us over the years that are yet to be answered. Why the 38? We don't know. Maybe the decision that this game should exist was the 38th decision that the planet overseer of our very own planet picked? What about the other 37 decisions? What were they? Are the reasons we have videogames nowadays just another decision made by someone, high there in space? Ah... mysteries, mysteries.
On all seriousness, it's a reoccurring gag we have been making ever since Ludum Dare 36 (which was our first game jam), where we decided to add the number of the jam's edition (36) to the game's name (Colossorama) as a joke. When we submitted Colossorama in Ludum Dare 36 it too was originally named Colossorama 36. However, when we updated the game with almost double the content, we figured we would drop the number in order to distinguish it from the original game jam version of the game - therefor just calling it Colossorama. We did the same with Hyper Holomayhem 37.
Basically, the number just identifies that the game was made during Ludum Dare 38, and whenever we update the game, we will drop the number. Hope that satisfies your curiosity!