That happens to be a question that should give a very broad answer. Technically, every person has a different hearing "capacity". This is assuming they can hear perfectly fine in both ears.
When a person first starts off music composition, often their ear adjusts to the headphones or speakers he/she uses. Let's say the person uses a pair of Sony MDR-7502's (even though they may be discontinued from certain areas) after using generic store-brand headphones. The Sonys have =2341"]this frequency distribution. Looking at that, you can see that it doesn't give much bass at all, and the treble is almost as weak. However, if those were your second pair of headphones, you probably couldn't tell anything was wrong because your ears had grown accustomed to those new headphones specifically. Now let's say you read some reviews online about some other headphones that you think seem better. Let's say the headphones you picked are Shure SRH240A. Those have =2801&graphID[1]=&graphID[2]=&graphID[3]=&scale=30&graphType=0&buttonSelection=Update+Graph"]this frequency distribution. You can see that there's a good amount of bass improvement and a bit of treble improvement. Specifics aside, it's obvious that the Shure SRH240A have a larger frequency response range than the Sony MDR-7502. Therefore, it is typically the case that when you replaced the Sonys with the Shures, you heard that the Shures sound better to your own ears, or at least different.
Let's say overall you are content with using those Shures for a good long while. Over that long while, in theory you would be able to hear more than when you had the Sonys. Why? Because the wider the frequency response range, the more previously-unapparent instruments in certain songs become more apparent and present. How far people are along this which-headphones-are-the-best-for-me path dictates his/her hearing "capacity". The better audio equipment the person has, the larger the number of instruments that can be easily observed.
That aside, the average person is typically at the moment in their headphone-purchasing path where they own and constantly replace generic earbuds such as skullcandies and iPod earbuds. Those people typically have hearing "capacities" close to that of the Sony MDR-7502 owners. Either too little bass and treble, too little bass or treble, too much bass or treble, or too much bass and treble. So really, hearing capacities are all over the place for the average person. Generally speaking though, the average person hears the lead sound first (typically vocals), followed by the bass, followed by the kick/snare, then the cymbals, then the harmonies, then the hi hats. Two exceptions are drummers likely hearing the drums first, and guitarists likely hearing the guitars first because of their inherent biases in favor of the instruments they each play. That's why you often see drummers air drumming to a song or guitarists air guitaring to a song. Commonly, the average person would probably hear 1~2 instruments at once on the first listen, and 2~4 if they were told to listen closely.