To figure out how much charcoal to use in your grill it is best to start out understanding a few basic principles. It's also important for us to be on the same page with methodology. For example, if you are lighting your coal with lighter fluid, you may use a different amount of coal than someone lighting with a charcoal chimney. It just depends on the exact time you begin cooking over your coals. It also depends how thin you spread your coals out. As an example, 20 lit briquettes will burn your hot dog if you cook over them 15 min after lighting but will barely cook your hot dog if you cook them 60 min after lighting. So to begin answering the question of how much charcoal to add we need to establish a few basics:
For all of the grilling recipes posted on this website, I have used my standard Weber Charcoal Chimney. The purpose of the chimney is to light the charcoals and to provide a means of keeping the coals in proximity while being lit. When using the Weber Performer (which has the small propane ignition feature to light charcoal) I fill my chimney either 1/4, 1/2, 3/4 or totally full - depending on the application. I then sit the chimney over the flame port and light it for about 2 minutes. There is no need to go longer than this and if you stick to 2 minutes you will be able to use your mini propane tank for a very long time. The goal is just to get a few coals lit and then the others will light afterwards. If you don't have a Weber Performer you can either use an olive oil soaked paper towel or some newspaper. It's the same principle. Just put the paper towel or newspaper underneath of the chimney (above the grates) and use a match to light it. After a few minutes the coals will ignite. Another question that pops up is when to dump the coals out of the chimney? Do you wait until you see flame pouring out of the top or do you wait until they are completely ashed over in the chimney? I like to dump my charcoal chimney as soon as I see flames starting to show over the charcoal. Thus, the top layer of coal is usually mostly black but the flame has risen up over top of the top layer of coals, as shown in the photo below. At this stage I will dump the coal in one pile in the grill. All the coals are not fully lit yet, so I dump them in one pile and will spread them out to where I want them after they have ashed over and look grey (about 15-20 min after the initial lighting).
A very simplistic way to discuss how much charcoal to add is based on how much you fill the charcoal chimney. You can imagine that filling the chimney 25% full will yield less heat than a chimney that is filled up 100%. To expand on this, here is my general rule of thumb for cooking various items at various heats using Kingsford charcoal:
Now clearly there are a lot of factors going on here. How you spread out the coals plays a major factor in the heat your grill experiences. If you dump a 50% full medium heat charcoal chimney in one spot and don't spread it out, you will have one isolated area of high heat right over where you dumped the coals. If you spread this 50% full chimney out over the whole grill area your heat will drop because the coals are more spread out. Make sense? So for my purposes, I mostly always use the following two zone grilling set up. I dump the lit chimney (see photo above) on the right side of the grill, and when the coals are ashed over I spread them out on the right side of the grill, leaving the left side of the grill with no coals (for indirect cooking). See the photos below. Roughly speaking, the above percentages will work out well for you if you follow this rule. The cooking temperatures are assessed once the coals are spread out and have mostly ashed over.
As a handy tool, use your hand to determine how hot your grill is. You can roughly approximate the temperature by how many seconds you can hold your hand over the coals. I usually hold my hand about 1 inch over the grates to do this little test. Count how many seconds you can hold your hand over the coals before you are in pain. If you can hold your hand over the coals for 5-6 seconds, you are dealing with low heat. 4-5 seconds - medium heat. 3-4 seconds - medium high heat. 2-3 seconds - high heat. If you can't hold your hand there for 1 second or less, you are not ready to cook because it is too hot. Clearly there is some variation here, especially with how each person perceives pain. However, the general person can hold their hand over a high heat for about 2 seconds.
Once you are comfortable with the above information try grilling something like boneless skinless chicken breast. Generally, this is a difficult thing to grill correctly, but if you understand your heat and direct vs. indirect grilling, it will become quite easy.
The type of grill you are using will also come into play. A smaller grill like a Weber Smokey Joe would scorch your steaks into a pile of black ash if you use a 100% filled charcoal chimney, so you need to factor that in as well. Also, some other grills have grill grates that can be lowered to come closer to the charcoal. The closer your grate is to the coal, the hotter your cook will be.