This may seem like a silly question but i'll ask anyway. Whats the difference between a 10 amp brushless speed control for speed 400 and below motors compared to a 20 amp brushless speed control for speed 400 and below as well.?

I forgot to mention. also, how do you know how much amp you should use for certain electric brusheless motor?

First, "Speed 400" motors are brushed motors. Mega does make brushless motors in a "400" size but they are not Speed motors. The amp draw of a motor depends on the pack voltage, prop size and pitch and gearing, if any. You can use an e-power spreadsheet to calculate the current. An ESC with larger capacity will cost you more and weigh a bit more.

The difference is how many amps the ESC will handle before the "magic smoke" escapes (the ESC burns out )

I would STRONGLY suggest D/L Motocalc. It is free for a 30 day use, then you have to register it ($35 ). If you put the information into the program (plane, motor, ESC, prop, weight) , it will tell you how many amps the motor will be pulling at various speeds with various size/pitch props. Find the highest amp draw and add 5 amps to it (for a safety margin ). That is the size ESC you need.

If you don't want to use Motocalc/Electricalc, use the size motor, prop, battery and ESC suggested by the manufacturer (Many manufacturers state the sizes of all this equipment for AVERAGE performance )

I feel that if you plan to stay in electrics at all, you should have either Motocalc or Electricalc to obtain the best results from your plane.