The fats are id'd by the number of bits used to count storage blocks.

Fat12 - 2^12 = 4096 ~ 2MB (512 byte/sectors) - limit of original floppies.

Fat16 - 65536 ~ 256MB with 4K/cluster (2^(16+12)), 
                2GB with 32K/cluster (2^(16+15)).

Fat32 (actually 28 bits) - 268,435,456 
   ~ 2^28 * 2^12 = 2^40 (1TB with 4K/cluster)

If default size block access size of a drive is 512 bytes/sector
  Fat12 - 1MB
  Fat16 - 32MB
  Fat32 - 1GB

Clusters
  To minimize record keeping by MS-OS,
    sectors are grouped into clusters of 2 to 64 sectors.
 
    And FAT table recorded the index of the cluster and the bios would 
      multiply and sequence through the multiple sectors of the cluster.

  All clusters on a partition are same size.

  Sectors of a cluster must be contiguous and on a single track.
    * If system uses c/h/s
    * Systems using LBA may allow clusters to span two tracks.

    When there are not enough sectors remaining on a track to make up
      a cluster, this is known as slack space and is lost (up to 30%).

  Limits
    A file needs a minimum of one cluster for storage.

    Different versions of DOS have different min./max sector/cluster ratio,
      Minimum coverage of storage device means large clusters.
      Large fat on small storage means large fat wasting space.