It's really a lot easier if you just make multiples of that item.
But if you want really items to be able to stack, like (Wood Sword x 5), then you could always make the wooden sword $ITEMSTATE1, mark the item as undroppable, and then add a new variable called %WOODSWORD or something. Everytime the player picks up a new wood sword, include the script:
%WOODSWORD := %WOODSWORD + 1
If they lose one of those swords, include the script:
%WOODSWORD := %WOODSWORD - 1
Then on your global page script, have this piece of code:
IF %WOODSWORD > 0 THEN
$ITEMSTATE1 := 1
ELSE
$ITEMSTATE1 := 0
Then in the item description, you can have something along the lines of:
"You have %%WOODSWORD%% wood swords"
and it will display how many the player has.