This is a trick question, since it can be interpreted in two ways; how many frames do I need for a video of a certain duration or what is the frame rate needed for a certain amount of motion to be clearly portrayed?
For both questions to be answered satisfactorily, we first need to talk about two very important properties of videos; interlacing and frame rates.
The first important choice when shooting a video is whether we will shoot it in interlaced or progressive mode. Interlaced videos contain two kinds of frames; those containing odd lines and those containing even lines. When displaying these frames consequently (first odd then even) at a high enough frequency, the viewer cannot distinguish the empty lines and therefore perceives the video as sequential full frames. Progressive (or non-interlaced) videos on the other hand, display full frames (all lines appear at each frame). From the description of these two types of videos, it can be...