i dont think theres a limit but personally i would probably not drink too much of it
you might be in danger of substituting water for coke zero which cant be ideal
i think those drinks can get quite addictive too
i used to drink 2litres of diet coke everyday many years ago and if i didnt have any in the house id get really stressed out
i decided to wean myself off and it was difficult id definitely become reliant on it , if not addicted
like everything else its good to do everything in moderation