How many blades in a perfect circle?

I think every ruler I've ever used for Dresden plates has a different number of blades required, based on the degree of angles used.  And that is the source of my confusion.

Notice anything wrong with this one? Too many blades!! This is what I get for doing mindless sewing on random UFO projects.  These purple dresden blades were in a zip lock bag on my cutting table so last night I thought I would just quickly sew them together.

Well "quickly" turned into this ruffled mess, some time online researching the ruler I used, and then tearing out 4 blades.  Then I stitched the sections back together. AND - with a permanent marker I wrote "20 blades" across the top of that ruler.

I also have some short purple blades ready to make into small plates. These will all be appliqued onto a solid background for a baby quilt. I'm thinking yellow or lime green for that background.