Smash Wii U didn't had a "DLC Stages" button, at least not among regular stages. It had a separate tab on the bottom
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And I'm quite sure they will do something similar in Ultimate, probably in the upper part of the screen
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Also I doubt that we will see characters like Isaac or Rhythm Heaven without an home stage. Since Brawl every single fighter had a stage for its series, the only exception being R.O.B. because he doesn't actually appears in the R.O.B. games, and instead uses old NES based stages like Mario Bros., Baloon Fight or Wrecking Crew, but since they are labeled as their own series they can't be used for R.O.B.'s background.
But considering how that count isn't centered makes me suspect that we will see more stages. Especially because 104 (103+random) can be easely put in a perfect rectangle 13×8. If that was the final count I can't see Sakurai passing on that opportunity to have a clean SSS.