Its Top 50 and not Top 20.

As for Ranking, it uses Ranked Battle and not Overall Power. Guaranteed for sure as when I started Plunder 2.0, I was ranked 60+ in Overall Power, but 40+ in Ranked Battle. While my group mate was ranked 60+ in Ranked Battle, and 30+ in Overall Power and he could not do Plunder 2.0.

That's a bug that's already been reported to the mods here since 2.0 rolled out. However, it hasn't been reworded. But its definitely Top 50 instead of Top 20.