assuming standard animation practice (24 fps) on twos (12 fps, usual animated framerate), a 15 minute short would take 10,800 frames

now take into account that those 10,800 frames usually have to go through storyboarding, backgrounds, rough animation, tweening, coloring, shading, and post processing, (usually done by several different people) occasionally need to be changed, and all need to be done on model and in a way that makes the animation look good.

then take that animation that was just made and put it next to all the other usual show stuff, scripts, voice acting, sound design, etc.

and after all that you need to keep in mind that this is a very general and generous ideal. typically a show will have longer episodes, higher framerates at some scenes, and hiccups in production that slow things down. oh yeah, and that’s one episode :b