Why need more aperatures in microphone array?
The more mics you have in the array, the better noise rejection you will have.  Much of this improved noise rejection comes from a "tightening" of the beam that you are forming-- so you also get better directionality.

Think of it like this:  Let's say that you have two mics, about 1 foot apart, and a person directly in front of the mics.  Because of the direction, the two mics have the same delay.  Now, place a noise source 90 degrees to the side.

The mics are 1 foot apart, which is roughly 1 wavelength at 1 KHz.  So if our noise source is spitting out 1 KHz, then that will be picked up in the mics just as clearly as the person in front of the mics.  One mic will pick up the 1 KHz exactly 360 deg phase shifted from the other, but we need 180 deg phase shift for perfect cancellation of the noise.

This 1 foot mic separation would also pick up harmonics of 1 KHz, too.  So 2KHz, 3KHz, 4KHz, etc. will all be as clear as the person talking.

If you went from 2 mics to 4 mics, each placed 0.5 feet apart, then our 1 KHz noise will be perfectly cancelled out, but we still have a problem with 2KHz and it's harmonics (4KHz, 6KHz, etc.).  

If you created a polar plot of microphone (and mic array) sensitivity (degrees off from center vs. sensitivity vs. frequency) you would see what we call side lobes.  In my example, there is a side lobe at 90 degrees, but the actual location of the lobe(s) depends on mic spacing and the frequency of interest.

In an ideal microphone array there are lots of mics placed in a 2-D array.  The overall size of the array will determine how directional the array is at low frequencies, while the number and placement of mics within the array determine the location and magnitude of the side lobes.  The spacing between the mics should not be regular (for example, every 12 inches) because that tends to create large side lobes.  Instead you want some irregular spacing (3 inches between these mics, 4 inches between these mics, 6 inches, etc.). 

Of course, no microphone array can be perfect.  An ideal (but not perfect) one for the human voice frequency range might be 10-20 feet across and contain 100 to 200 mics.  Anything less requires a compromise on performance.  It is up to you to figure out what compromises you can live with, and design your array accordingly.