We have a 10x12 shed with a turbine vent on top, wind activated. We also open a window in the summer otherwise the vent doesn't do a whole lot.

Vents come with screens built in.

Just cut a hole in the roof and slide the top under the shingles and the bottom over.

One should be enough.

A couple of 8" x 16" vents near the bottom of the wall and a couple of others up high in the gable ends should provide decent ventilation. You can find the louvered, screened vents at the big box DIY centers.

Thanks for your responses hendricus and randy. It sounds like I'll go with the louvered ones. And I didn't know they had screens, so sounds good. : )