We have 14 sets, though some of those shutters aren’t very long, and we will use five cans. I made a u-shaped template to put on the bottom half, then flip and do the top, but she ended up just using a single piece of cardboard, worked around the perimeter, and didn’t overspray al all.
I used Krylon and it’s about $9 a can, but I searched and Rustoleum makes a clear gloss for $4. I’m not sure if it’s uv resistant or not.