Potential Number of PKMN in the Dex...

[Kieran]

I won't mind the small buff.
Tbvh Alola was the first time I ever bothered getting the Shiny Charm (When you have such encouraging Rotom, you can't help it too!).

 

[OmegaRuby and AlphaSapphire]

I think that they'll place the Ultra Beasts close to each other. If not then I can see the new ones coming after Necrozma (as it, Solgaleo, and Lunala are UBs themselves) but not Magearna and Marshadow (in the regional dex that is).

I won't mind the small buff.
Tbvh Alola was the first time I ever bothered getting the Shiny Charm (When you have such encouraging Rotom, you can't help it too!).

I hope to complete it this time with Ultra Moon :).

Edit: I just thought of a reason for why SM lacked a national dex. That reason is that GF planned to add new mons and wanted the legendaries and mythicals to remain at the end of the dex. For example, Necrozma isn't really Pokemon#800 in the national dex like we all were expecting/thought.

 

[Hikamaru]

400 seems fine enough for me! :o I wonder what additional Pokemon they're bringing over to Alola this time around?

Well, based on trailers (another new one just came out) here's the older Pokemon that so far seem to be getting added to the New Alola Dex:

Mareep, Flaaffy, Ampharos, Mantine, Houndour, Houndoom, Larvitar, Pupitar, Tyranitar, Electrike, Manectric, Tropius, Mantyke, Zorua, Zoroark, Mienfoo, Mienshao, Larvesta, Volcarona, Inkay, Malamar.

Edit: Dedenne and Heracross seem to have been confirmed in the new Japanese trailer as well.

 

[Sabrewulf238]

Well, based on trailers (another new one just came out) here's the older Pokemon that so far seem to be getting added to the New Alola Dex:

Mareep, Flaaffy, Ampharos, Mantine, Houndour, Houndoom, Larvitar, Pupitar, Tyranitar, Electrike, Manectric, Tropius, Mantyke, Zorua, Zoroark, Mienfoo, Mienshao, Larvesta, Volcarona, Inkay, Malamar.

Edit: Dedenne and Heracross seem to have been confirmed in the new Japanese trailer as well.

I think it's been said already but you could make a strong case for Remoraid and Octillery since Mantyke needs Remoraid to evolve into Mantine.

Also, I'd love to see Mienfoo/Mienshao added to the regional dex. That's one Unova pokemon I've wanted to get around to using. Larvesta too but it evolves at a really late level which is annoying.

 

[Hikamaru]

Also, I'd love to see Mienfoo/Mienshao added to the regional dex. That's one Unova pokemon I've wanted to get around to using. Larvesta too but it evolves at a really late level which is annoying.

The English trailer covering Necrozma's new forms did show it using Solgaleo and Lunala's signature moves and the Pokemon shown being hit by the moves was a Mienfoo.