help with checking a certain number of an item in inventory

I want to keep a stack of say cobblestone in the players inventory if they have this item ive made in it.  
so I want to check if the player has less then 64 cobble stone and if they have less then add up to 64 cobble in. if they already have cobble then do nothing. 
I thought i had it figured out but i cant seem to put the procedure together that would make it work.  Any idea how to do it