Unfortunately You can only guess..
Im flying mostly on OS and from my experience the airlines are pretty good with estimating how many seats they can "sell twice"
One time a Fokker 100 flight was overbooked by 15 people, and it ended up taking off with 1 free seat..
my other major overbooking situation was on a EWR to VIE flight with 2 seats for sale in business class , while economy was overbooked by 18 people.. And they ended needing only 4 VDBs..
if you seek to get bumped for the compensation I would advice you to ask at the check in counter and proactively offer your seat..usually the staff is quite thankful in such cases