Ratio of key frames to in-between frames in anime
The answer is relatively simple when looking only at the ratio (ie. not at the number of keyframes over the course of an episode or minute of anime).

Since keyframes are used at the start and the end of a smooth transition, each movement, or transition, only has 2 keyframes, regardless of how many people or objects are moving over a period of time.

From clicking around on Wikipedia's Keyframe page we get that a movement can be drawn in about 12 frames.
With frame 1 and 12 being keyframes, the rest are inbetweens, resulting in a 1:6 ratio of keyframes/inbetweens, where the ratio can move to 1:12 if the movement/transition takes a full 24 frames, or may even drop down to 1:3 (for example) if additional keyframes are introduced successively in order to complete different motions starting and ending at different times.