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.