Let's take a look back at the Frieza saga where we had lots of different races 'employed' as soldiers of the emperor Frieza. At least 79 planets were under Friezas control. The minions of Freeza - also the Saiyans - were sent on missions for decades in order to extinct the inhabitants of other planets to sell the place later on.
Therefore, of course, many races died of course but it's not like the Frieza army ran out of work. I mean we are talking about the whole Universe 7 here. It sounds even way too less for a galaxy. Of course also Buu and Beerus were responsible for destroying entire planets and civilizations, but on the other hand the Kaioshin is responsible for the creation of new worlds - it seems like he failed in doing just that.
Or is it that the young Kaioshin doesn't know better? He was wrong and did not know things so many times. Like being unaware about the strength of the Saiyans, the by Babidi moved location of the 'egg' of Majin Buu, the Potara fusion or even recently the existence of Zen-Oh.
He is still a young Kai but these examples makes me believe that he is also wrong this time. He even went checking for other strong fighters for the tournament.
Also, if we would include 'Dragon Ball GT' - if that's even possible, but that's another topic - we get to know way more planets like Imegga, M2 and so on. You can say about the third Dragon Ball series what you want, but it gives a great view about the universe. Something which started at the Saiyan arc back in DBZ where we got the first information about what's out there in outer space.
If Super's interpretation is that Universe 7 only has 28 planets with mortals sounds for like: "Don't make it too big, we have 11 other universes." Anyway, these are my first thoughts after that revelation, also check out the video. Hopefully we will know more very soon.
