At any instant how many teeth in a (not new) 52 chainring are actually doing the chain pulling when torque is being applied.
Depends how well the chain and chainring mesh. Assuming perfect alignment, exactly half the teeth are engaged.
In the real world, the chain will be somewhat elongated. So your worst case is one single tooth is taking all the strain. At this point, wear is accelerated.
You should use "27 teeth" and "1 tooth" as inputs for your needs.
EDIT ChrisH calculates that assuming a 52 tooth chainring and an 11 tooth rear cog, there will be an 11 degree angle between the horizontal and the chain, on either side.
So 22 degrees over 180 gives 202 degrees of the chainring are touching the chain. 202/360 shows 56.1%of the chainring, which is 29.18 teeth, so 29 teeth in contact with the chain, highest possible value assuming a 400 mm chainstay length.