FREE RANGE PEKIN

I think ten would be the upper limit, if not less. Pekins are really rough on a yard with their size and the amount of waste they cause. If you were going with all pekin, I would probably do 6-8, but if we are talking about smaller, less destructive breeds, you could probably get away with 10. But then I probably overestimate how much damage ducks cause to a yard because I am overly cautious. I'm currently in a similar situation with trying to decide how many birds my backyard can sustain. I have way too many

Your yard is 10 yards x 35 yards. Three feet in a yard, so it's 30ft x 105ft. That means your yard is 3,150 square feet. An acre is 43,560 square feet, so you have 7.2% of an acre for your yard. I personally think that you might be alright with ten ducks, depending on the breed. If you are really worried about vegetation destruction, maybe start with less and think about adding on later if all goes well.