$ModuleNumber

$ModuleNumber gives the current serial number to be used for local variables that are created.

$ModuleNumber is incremented every time Module or Unique is called.

Every Mathematica session starts with $ModuleNumber set to 1.

You can reset $ModuleNumber to any positive integer, but if you do so, you run the risk of creating naming conflicts.

See The Mathematica Book: Section 2.6.3.

See also: $SessionID, Temporary.