I'm looking for a guide on 2D animation. I'm very new to it. Specifically what I want is to find the minimum amount of sprites needed in an animation cycle to simulate fluid/semi-fluid movement.

ex) Maybe a highly detailed walk cycle has 10 unique sprites in its spritesheet. But a minimum acceptable walk cycle that tricks the eye well enough can be done with only 5 unique sprites.

Now I want to apply this knowledge to other visual animations such as a wizard's cape flowing in the wind, or a flaming fireball (as opposed to just a static fireball image that translates across the screen).

Could someone recommend a guide out there that will put me on the right track to attaining the skills needed to do some basic 2d animations.

Don't have a specific guide for that for you but what I've noticed doing 2D animations is only drawing every 2 frames of the "full" animation which makes the animation fluid enough, at least for me

example a door what I drew, skipping every second frame from a "full animation"

I spam this ebook a lot since it's the only resource I found on the subject. There's a chapter on sprite animation:
http://www.widgetworx.com/book/ (download link at the bottom of the page)

the minimum amount of sprites needed in an animation cycle to simulate fluid/semi-fluid movement?

There isn't a definitive answer. The more unique drawings you have in a sequence, the smoother it will look.
The ideal amount would be the rate of the display device (usually 60 Hz), so for every flash of the device you have a unique drawing.
But this would mean 60 unique frames for a single second of animation and that's unfeasible.

Some cheap cartoons can get away with 8 unique frames per second.
https://en.wikipedia.org/wiki/Persistence_of_vision#Cartoon_animation

http://9gag.com/gag/aGDpnRz/15fps-vs-30fps-vs-60fps

If everything in your game uses the same (low) framerate, after some time our eyes get used to that and you can at least get a feeling of motion.