Check out this tool.
https://meraki.cisco.com/public/mv-site-survey/index.html
The problem is the answer is not black and white. The larger the area covered the less detail you get. How much detail is required depends on what you are trying to achieve.
For example, I often have cameras on the entrance and exit focusing on a very small area so I can get excellent facial photos. For the rest of the area, I might use a camera to cover a larger area because the only thing I want to see is a persons movement.
You have vehicles - so you might be interested in being able to read number plates. You are going to need the camera focused on a very tight area to get that.
Also, no matter how many cameras you put in, you'll find you could always have used more. Always.
