There are several sources of artefacts:
1. Diggable, one per continent, per previous era
2. Artefacts from overbuilding (you have some chance to get them)
3. Artefacts from natural wonders, which anyone could dig once per wonder (after discovering Natural History mastery)
4. Artefacts from random events
5. Artefacts from doing research after discovering Hegemony (one per continent)
6. Artefacts from future civics
etc.
Overall it's possible for the game to finish without anyone gaining enough artefacts, but normally it's reachable if you focused. I had a game where I actually completed cultural legacy path with artefact gotten from future civic (although this game ended with score victory soon after).