depends on the animation, complexity of the model, and how much video memory your willing to use up for the model ( every animation frame takes up memory)

any animation that requires a 'rotation' like motion ( like a wheel or rotating an character limb around a joint) will usualy take more frames because vertexs can only be 'rotated' less than 30 degrees between frames, else the mesh deforms.

soem animation examples:

a pistol being fired: you could possibly do this in as little as 2 frames, for an automatic pistol

engine piston: the piston itself would need only 2 frames, but with a connecting rod motion included at least 4, add the crank shaft and you would need 12+ frames

an human arm. starting at hanging from the side to straight above the head.

Natural motion is most people rotate the elbow 90 degrees first when raising the arm,
then the upper arm rotates(around the shoulder about 110 degrees (depending on flexability),
then the upper arms contiues to rotate to 180 while the lower arm unrotates back.

so add that up its about 4+ , 4+ ,4+
so at least 12 or more frames