How to read frames which are separated by a specific number ? [ffmpeg]
The direct way to do this is by using the concat demuxer, where the text file only mentions the selected files. Besides being a hack, you no longer get to use the image2 options. The better way is to combine your present commands.

ffmpeg -framerate 20 -i frame-%d.png -vf "select='not(mod(n,20))'" -c:v mpeg4 -y movie.mp4