57
Crappy D20 [Rule]
(files.catbox.moe)
After 250 rolls, 1 popped up 27 times. Or roughly 11% of the time.
I did this same test with all 17 D20's I have and 2 D20's from people in my DnD group.
This one was far and away the worst though I had more than I'd like with averages under 10.
Edit:
Here's the stats for the ones I tested
This excludes the data for the 2 D20's from the people in my DnD group as they didn't want that data shared outside of the group chat.
Now I think you should test if averages hold consistent between same sided die from different manufacturers. I think another poster is right that the shape matters, so if the die from the same caster are molded roughly the same, then they’ll present with similar averages.