distance between flush clips

I have a wooden storm door ( 36 X 92 in) with a wooden framed insert panel. Into this insert panel sits an insert panel of plexiglass with an aluminum frame. (I have another one with screening for summer.) Swisco flush clips (with threaded rivnuts and thumbscrews) are the best solution for me.

My question: The panel dimensions are 24 x 53 inches. At what distance/ how many flush clips should I install?

Thank you.