The latest Pledge number is stored in a Sequence (an Oracle db object) called PLG_NUMBER_SEQ and it is incremented as it is used in Batch Detail or other places where a new Pledge number is required.  Here is an example of the settings for ARC4:
-- Create sequence ARC4
create sequence PBDS.PLG_NUMBER_SEQ
minvalue 0
maxvalue 1010
start with 61
increment by 1
cache 20
1.  The maxvalue controls the highest value a Pledge number can go.  Typically, this value can run up to 999999999 or such that will yield non-repeating values. 
2.  The cycle parameter has been turned on in this example, which means that once it gets to the max number, it starts again from the beginning.

The latest Pledge number can be reset manually in the front end, here, in System Preference (under Entry/Gifts):

User-added image

This assumes that have the setting "Automatically Assign Pledge Numbers?" set to 'Y'.  More on that option can be found at this link.

More information on resetting the Pledge number can be found at this link:  Resetting the Pledge Number.  This option does allow you to set a new Pledge number to increment from.