"The first proposal was completed in December 2015, back while we were still working on DLC for the previous title. It wasn’t until later that I assembled a team and began development on this new title in earnest. "
http://n64josh.com/super-smash-bros-ultimate-development-details/
So a bit less than 3 years of work, assuming a generous January 2016 start and October 2018 end.
Considering the following:
- So far, the unique newcomers introduced have been to meet long-time fan demand
- A significant amount of time was spent on bringing back characters and stages
- An entire new category of character was created for 1:1 clones that explicitly less work put into them then other characters, making the roster seem bigger with less resources.
- Sakurai chose not to revamp its art-style or its engine completely
- Many characters have add changes applied to them, instead of direct translations
- In the world premier, Sakurai came out and emphasized telling people not to hope for many new characters.
- The "hype cycle" for Smash Ultimate is half a year in length.
With this in mind, I find it highly unlikely that Sakurai's going to be hiding a huge 10+ roster up his sleeve.
Unless, of course, the majority of the newcomers are going to be echoes which take much less resources and could pad out the roster a bit more, but those aren't what most people consider "full" characters.