Considering legendaries account for a little under 10% of the national dex currently, and they tend to be popular when the games aren't so oversaturated with them that they aren't special, it would be weird if they literally only brought back one.
I feel like a dozen would probably be too many, and I'm happy a lot of them are getting cut (especially the mythicals, which have a serious problem with event move bloat), but I'd like to see a few more. Specifically, I'd like some of the lesser used ones brought back and given an opportunity to shine in metas without things like Landorus, Heatran or Primal Groudon overshadowing them.
I don't think there is any precedent for non-legendaries coming after the box legendaries in the regional dex, unfortunately.