My guess is that the artifacts are the objects you pick up throughout the realms (not the item count items). I found: soda can and milkshake (toward the end of the Regular House sections), three separate "memories" (ruined house 1), pumpkin and tuna can (abstract brutalist section; the tuna is hidden in a high vent), empty pill bottle and full pill bottle (restroom in airport). Possibly the T-shirt you pull out of the closet near the end of Regular House also counts, but I think that one's just a "switch" to progress.