copying formulas

first off that is NOT a number that is a string value referring to a Sheet Name.  There is no default built in way to increment that Name.  That said here are some work arounds you can use:

a) INDIRECT function:

=INDIRECT("'" & 725+xxx & "'!$G$64") where xxx is some function that will output what you need to increment by.  For example if you are copying down a row starting on row 14 it could be ROW()-13 so it would look like:

=INDIRECT("'" & 725+ROW()-13 & "'!$G$64")         or      =INDIRECT("'" & 725+ROW(G1) & "'!$G$64")

the same could be done for COLUMN() or even SHEET() if this will be copied across sheets.

b) TOCOL , TOROW, VSTACK, or HSTACK functions:

so if you want a column or row of these values and presumably those sheet names are of a consecutive sheets, then you can simply use:

=TOCOL('725:799'!$G$64)

if they are NOT consecutive sheets but follow a standard pattern like '725', '725old', '726', '726old',... you can then do the same thing but FILTER or DROP the unwanted values.  For example if they are every other sheet you could use:

=DROP(WRAPROWS(TOCOL('725:799'!$G$64),2),,-1)