You don't have to do that many coats, you just need to get full coverage, HOWEVER, what I tend to do is this. I make sure I gain full coverage first, then I let it fully flash, typically hours and sometimes overnight. This typically takes 2-4 coats but I almost always do 3 regardless of the color unless I know for sure it has full coverage because I have this issue where I tend to miss a spot, which typically is the bottom of a door edge lol. Sometimes that third coat is me walking around with a sun gun just blending in spots, checking edges etc etc etc. I recommend doing this with the lights off and a sun gun in hand, I found that the sun gun is a life saver for me in finding spots I missed.
Then after it sits all night I will go over it with the foam sanding pads to remove any trash from the paint job, typically I just scuff the whole body down, I then blow it off very well, and tack it multiple times. After sanding, I always put down two more coats to cover up all of the sanding scratches. These last two coats are always very thin coats, I call them orientation coats.
Let's discuss the difference between the two phases. the first round of applying base is strictly for coverage. I need to make sure it has coverage and that's the primary goal. If it's a metallic and I have some stripes or the metallic isn't as I want it to be I don't even worry about it. because I know I am going to sand it and re-spray anyways. I find that with auto art the last two coats will correct any blemishes anyways, so I don't worry about it. Then my final two coats are as I would call my orientation coat, the whole purpose of these two coats is for me to apply them as even as I possibly can and make it look as good as I possibly can, in mind I don't have to be concerned with coverage because I already achieved that, I find that it makes it easier on me and even new painters to separate the mindset between the two operations when trying to get the best possible finish you can get. I also don't feel like those second to coats put on that much additional material as it's more of a medium application. Please make sure you get all of those sand scratches covered in this phase, this is typically why I do two coats here, one would actually probably be enough. But, when painting a expensive car I like to be over cautious.
Another advantage I see to this application process is that you can let your first round sit overnight and that gives it a good amount of time to flash off all of the solvents, typically on my second round I do not wait as long, I give it roughly 2-4 hours depending on the situation and go to clear.
In a collision environment I tend to do three coats, after the second one I tend to really look it over, scuff any trash out and then do a third coat to make it uniform and then just go right to clear, very different scenarios and really it can be applied to any job to achieve the results you need.
I find the color makes a big difference in the application process I choose as well, If I am doing a black car, I will only do two coats and let it sit overnight, then scuff and apply one more coat.
If I get a tricky silver I may apply 2 coats on my orientation round and then if I see a spot I don't like I may blend out a extra coat in the stubborn area.
So I guess I'm just saying I struggle with the how many coats will I need to apply question because everything above is considered when I am painting a car.