The tags listed here also include the ones from the poster (object) inside this pic (sitting on lap, yuri).

When it comes to the headcount, should I ignore the poster and keep 2girls or change it to 3girls including the poster?

FineAdditionToCollection said:

The tags listed here also include the ones from the poster (object) inside this pic (sitting on lap, yuri).

When it comes to the headcount, should I ignore the poster and keep 2girls or change it to 3girls including the poster?

Inanimate depictions of characters using a medium like posters, paintings, and figures are usually not included in the character count.
More information over at tag group:character count.