How to Get Access to Ancestry Feats
https://leagueofaroden.com/2021/03/29/access-to-ancestries-explained/ https://www.reddit.com/r/Pathfinder2e/comments/v0z3j3/can_you_grab_another_races_ancestry_feat/
As others haven't yet covered;
You can get the Adopted Ancestry feat either standalone as a General feat or as part of another feat. Note that being granted the same feat twice does NOT have any effect unless the feat specifically mentions "you can take this feat multiple times".
Sources of Adopted Ancestry outside of the General feat (again, you cannot, RAW, gain a second adopted ancestry via these feats plus a general feat, they do not stack):
Halfing 5th Level Ancestry Feat (Common): Cultural Adaptability
Hobgoblin 5th Level Ancestry Feat (Uncommon): Runtsage
Skeleton 1st Level Ancestry Feat (Rare): As in Life, So in Death
Anadi Heritage (Rare): Adaptive Anadi
One way to get access to a second set of ancestry feats for a total of 3 "ancestries"? You can take the Half-Elf or Half-Orc heritages as a human by default, or follow the optional suggestion printed in the book; allow half-elf or half-orc as versatile heritages for other ancestries (See "Other Halves" here). This does infact stack with "Adoptive Ancestry", as their wording simply states you gain access to both your inital ancestry and the half-ancestry of your choice.