They are close to zero (I think).
Most McDonalds (I know of) either have free refills or provide pre-filled cups.
So this mainly prevents you refilling "to often" or with "to old cups".
The huge majority of people will not "refill to often".
But homeless people might try try to get some extra drinks by refilling using old cups or cups from other people.
But they also don't really have the money to buy additional drinks.
So no relevant amount of not sold drinks are missed out on.
At least IMHO and with the kind of McDonald I have been in.
EDIT: Cleaned up sentences.