You need to do a search on "stereo vs surround sound". The unit driving the speakers in a stereo system has 2 output channels - left and right. If you have 20 speakers, 10 will be driven by the L channel while the other 10 will be driven by the R channel. All the L channel speakers will be on the left side of the car and the R channel speakers will be on the right side. If you mixed them, you would not have what is called stereo separation. A properly set up system will have you perceive sound as coming from the left, center, or right (even when there are only 2 speakers). So even though there are speakers in front, to the side, and behind you, a stereo system can only change sound from side to side because all the speakers on 1 side of you will emit sound.
Surround sound - let's take the 7.1 form. We'll ignore the .1 because that is the subwoofer channel handling low frequency sound and it is non directional. The 7 represents the 7 channels that are being output (this is similar to going to a movie theatre where you hear sound coming from front left, front center, front right, left side, right side, left back, and right back. So the unit driving the speakers has 8 channels to handle powering the 8 speakers. If a train passes you on the right, the sound starts front right, goes to right side then right back so you hear the train approach, come beside you then go behind you. Atmos adds verticality so you can sense sound above you. So if the left rear cross traffic monitor alarms, you hear the speaker behind you on the left. In stereo, you only hear it is on the left side...assuming they enable notification like that.
Why are there 12 or 21 speakers? Separate speakers handle high, mid, and low frequencies and they are spread throughout the cabin.