So, I wondered how many legal stages we would have in Smash Ultimate, so I decided to look at all the stages that have been legal in the past to get an idea. For this list, I'm counting counterpicks as well.
-Battlefield
-Final Destination
-Dream Land
-Hyrule Castle (Smash Wiki lists it as a technically legal stage due to players sometimes picking it via the gentleman's clause)
-Fountain of Dreams
-Pokemon Stadium
-Yoshi's Story
-Castle Siege
-Delfino Plaza
-Frigate Orpheon
-Halberd
-Lylat Cruise
-Smashville
-Yoshi's Island (Brawl)
-Prism Tower
-Arena Ferox
-Town and City
That's 17 legal stages already, not counting the possible legality of any new stages we haven't seen so far.
For perspective, Smash 64 had 1 legal stage (technically 2, see the Hyrule Castle clause), Melee had 6, Brawl had 10 (and even then that was just because the stage list for Brawl was in contention a lot), 3DS and Wii U had 6 each.
If you count the stages that were only illegal because of Meta Knight (who has obviously been toned down since Brawl and is no longer a deciding factor in banning stages), then you have 3 extra legal stages from Brawl:
-Brinstar
-Pokemon Stadium 2
-Rainbow Cruise
For a total of 20 legal stages in Ultimate, and 13 legal stages in Brawl.