The actual FAT table is 4 bytes per entry.
FAT32/8=4
The number of entries depends on the cluster size 512,1k,2k,4k,8k,16k,32k, and 64k may or may not be allowed.
((Total size /cluster size) + (3 reserved codes)) * 4 = 1 copy of FAT in bytes. Sectors are usually 512 bytes,sometimes 4k, and it is round up to the nearest 512/4k byte marker.
Now multiply that by 2 as there is a backup FAT table.
Note: FAT tries to keep a copy in memory at all times, therefore microsoft has artificially limited its size to prevent memory exhaustion.
4 billion entries (theoretically possible) *4 bytes =16 billion bytes or 16gb of RAM just for the FAT table.