+1

I've seen an upgrade list with 45 people on it (yes, I connect through ATL sometimes), and a flight that went out with empty seats. Heck, I've even seen an upgrade list over 30 people on a CRJ-700, also with seats still available.

Upgrade list only tells you how many people will be boarding with you in Sky.

However, if you see a standby list with over 20 people and 0F, 0Y available, that might indicate oversold, but no guarantees. It might just be IRROPS on the flight before that one. I've been on standby for a flight that had over 25 people on the standby list, and they were not offering VDBs.