Daedelus

Tie them on to the lower of the two guard rails with the ends just clear of the water for most pontoons, then if you need to alter them...

Friend had a 42foot Jeneteau and they numbered the fenders and had a sheet saying where each should be placed on the boat. She took...

At least 10 of reasonable size, I near bows, 1 near stern and 3 spread along midships bulge per side. Any more is a bonus.