can anybody tell me whats the max recommended number of cards to have in the card box when using the clip?
The answer is that the number of cards will vary depending on the deck.
AS an example, I can get a standard set of Bike in the Porper clip with a few additional cards in the box but a standard deck of Masters with the Jokers removed won't fit into the clip. Ghosts are also a tight fit whereas Tigers are just OK but you won't squeeze any extra cards in.
Hope this helps.