Nobody can really tell you "how much cotton." Partly because it depends on the cotton and the RTA. I can tell you how to gauge it in general terms.
Essentially you want the cotton just snug enough, going through the coils, that you feel a bit of resistance pulling it through them. It doesn't have to be exact, but either too much or too little "pull resistance" will mean less juice delivered to the heating area. It will take practice, trial-and-error, to learn just what is the right amount of pull resistance.
Then comes the fit of the ends into the juice wells. In general terms you want enough to fill the juice wells, but not snugly. This can actually be harder to get right than the amount going through the coils. Usually, with the right amount going through the coils, there'll be too much for the juice wells, so what you do is comb it out to make it thinner and trim off the excess.
When you get the thickness of the cotton going into the juice wells right, it should actually have a bit of "wiggle room." By that I mean gently prodding the cotton allows it to fairly easily move around in the juice well. It may seem to be too little, but remember: When it's wetted it'll swell a bit. If the cotton is tight in the juice wells when it's dry, it'll be constricted when it's wetted.
Then there's how far into the juice wells the wicking has to extend. That varies from RTA-to-RTA. For example: With the Zeus Dual and Zeus X RTAs I'm using, the wicking should just barely tuck into the juice wells. Too long wicking into the juice wells will reduce flow on these RTAs.
The best thing to do may be to look up YouTube reviews and build tutorials on the RTA for which you're building.