Set Label Text as Custom and click the dropdown list to see Custom notations.

Notations can be combined, e.g. h$n$ to show #1, #2, ....

a$ (default) – enumerate lower case letters
A$ – enumerateupper case letters
r$ – enumerate lower case roman numerals 
R$ – enumerate upper case roman numerals 
n$ or # – enumerate numbers
h$ – represent #

Graph## – Graph01, Graph02, ...

hsn$ means #1, #2, ....

Labtalk Script example:

merge_graph labeltext:=custom labelcustom:=h$n$;

Tip: Click the > button next to Dialog theme and choose Generate Script, executable Labtalk script will be created.