If i run multiple of them, do i have to get 2 power more to win for each?

Lets say i run Rangers and Builders, do i need 17 or 19 power to win?

You need two additional power for each of them.

I´m actually not sure why they splitted the effects of the "wildling" and the "nightwatch" themes on many different agendas. I guess they are planning or probably will release with cp # 4-6 from the latest cp circle further cards like wolves of the north.

Orcish said:

Yes. It's no different from putting two attachments that say "character gets +2 STR" on the same character. Each one is resolved separately, so they add up.

Two "The North" Agendas would mean +2 @ for a total of +4 power. You would need 19 to win if running both The Rangers and The Builders.