I need to buy a 4s 650mah Lipo battery, but I am not sure what c-rating to get. This is what my esc is: Continuous current: 20A
Peak current: 25A (10S)
BEC output: no
Input voltage: 2-4S
Main control chip: 48mhz EFM8BB2
Firmware upgrade: Supports Dshot600/BLHeli_S/Oneshot125
MOS: 3*3
MOS type: vs3610ae 30v64a

So, I am not sure if I should get a 65c or 75c.
Thanks.

To get the continuous current output capability of the battery, you just need to multiply the amp hour rating of the battery by the C rating.

So in the case of a 650mAh its

$$ 0.65 * 65 = 42 $$

So 42A continuous current out

But to know how much current you need you will need to check your motors and prop combination to know how much current each one draws to get a battery around the same.

You donâ€™t necessarily need anlipo with a continuous current rating that exceeds the max current of your motors as realistically you will not be flying at max throttle all the time.

According to the formula: max continues Amp drew (A) = battery capacity (Ah) * discharge rate, so the 65c is 42 A, 75c is 49A.
The continuous output of battery current needs to be greater than the maximum continuous current output of ESC, so in theory choose a 650mah 4s battery, 65c and 75c is both compatible.