Depends!
My first major collection was pokemon and most people in the community I belonged to, including myself, focused on one (or a few) specific pokemon/pokemon line as opposed to pokemon as a whole. Not only because pokemon has a massive, MASSIVE amount of merchandise for it, but because it's fun to focus on your favourites. In those cases I was definitely a completionist collector but only for the 'mons I focused on.
I sort of do that with ponies. I collect G4 Rarity and want to have at least one of all her unique "3D items" (so not stickers or anything like that) and trading cards, and also certain sets of G1s instead of all G1s. But the more G1s I get, the more I fall in love with them and want them ALL.
I also collect anime/video game figures but I'm far more selective with those. Some series I like have tons and tons of merchandise, a lot of it very expensive. I'd rather just have two or three high quality pieces than thousands of little trading figs haha.