i have this eq already how many rca cords do i need to install this and will i need a four channel amp to hook it up to

We need more info about what kind of head unit and amplifier you have so we can help with how many RCA's you will need. Let us know. Thank you.

I have 3 set of outputs on my head unit, but only 1 set (plus an accessory set) of inputs. Do I just use the front out from the head unit to the EQ, and it can do front, rear, and sub out from just the 1 set?

You will lose front and rear staging and fader if you run the front RCA's into the EQ.  You should run the sub RCAs directly to the subwoofer amplifier.

The volume knob is labeled as "min" being all the way right, and "max" all the way to the left.... isn't this backwards?

if it refers to the input voltage mA would be backward

i.e. set the gain to a minimum when you have max signal output from your source

I have the eq hooked to my sub amp and my mid amplifiers it sounds good but when I push in the aux button the subs really sounds good but I can't hear my mid range speakers,I have tried everything,what am I doing wrong I want the subs to sound like that all the time

When you push the AUX button input signal to the mids will replace by the AUX input 

- if the signal is stronger for one amp, the output will increase. 
possible fix by adjusting the gains.