I would say around 5 to 6 as the minimum

Heres a pic of my 1st tom ever, he went down on a forty yard shot so I stood over him having a celebratory smoke waiting for him to die, after I put the smoke out 5min later he got up and tried to run away, as you can see a 5yrd shot gives em a jelly head.

if your asking this for patterning reasons, dont count pellets in kill area because that will change with every shot, count pellets in a ten inch circle, figure out precentage of shot in this circle, and look for gaps in the pattern. these are more important than kill hits when shooting at paper to figure out your loads.

shooting at an actual turkey, ive never counted but the more the better.

The truth of the matter is: It only takes one pellet to kill a turkey. I'm sure that isn't the answer you wanted though.