It really depends if the recording area is busy with motion or not.
Motion means higher bit rate and this means more storage will be required.
So in this case its needed to calculate the maximum amount of storage space you would need for one camera.
However, keep in mind that even if you set the bit rate to the lowest value 4, it might exceed once there is motion on it, and this is applicable the other way around, if there is no motion, bit rate should be lower and storage required would be lower, this is why we recommend when calculating to add 20% for redundancy.
The formula is: N*[(A/8)*24H*60M*60S]/1024 (M)
N is the number of cameras
A is the bit rate
(M) is Megabyte