The final dex will be 400, right now you can get to somewhere around 360 iirc. The biggest consideration for whether a Pokemon gets in the dex is whether I like it :P But there are other considerations, too, like type balance, giving good representation to each Gen, and in some cases, lore (I'd love to fit the Paradox Pokemon in there, but I'd have to bend over backwards to justify it lore-wise).
But yes, there are Pokemon (and, where applicable, regional variants) from every region, including Hisui.