I've played every first party and good third party games from zero to end (retail ones mostly). I know there are more than 50 of them. Around 50 GB games I've finished, as well and half the GBC games (some 30 or something). NES and DSiWare are the games I haven't played a lot, but my children play them for me. And at least 25 different eShop only games from zero to end, too.
It would be good if Nintendo had a way of sharing your gameplay profile easily online. When I first downgraded, I had to format my 3DS since it got soft-bricks every time before I formatted, otherwise I could have shown you photos of my old gameplay records. Not that you care a lot about it, but for backing up my statements with proofs.
But yeah, you got a point, I have finished 100% around a little bit more than half the games the 3DS install limit, so no need for 300 slots, but hey, I had a lot of variety for sharing with friends and family.