Pokemon XY shiny tag requirements.?

So I know that you need to "fill the national dex", but I want to know, to what extent?

For example, some Pokemon like Deoxys and Arceus are in limited supply, because they were only out for a limited time. Pokemon like Jirachi are still in circulation, because of the Pokemon Coliseum bonus disk. Celebi can be gotten from Pokemon Bank, but Mew is a rare find, since the events for it were extremely limited (limited time release, and only in certain locations).

Not to mention there's some unreleased Pokemon like Diancie and two other Pokemon within the National Dex following Diancie's dex number.

Basically, what's the minimum number of Pokemon needed to clear the flag to allow me (and perhaps other players as well) to get the shiny tag.