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$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.