'Max number of cards' has never been supported for repetition-spaced tests; we know Anki offers that option but we don't think they should, if you limit your maximum number of new cards and don't ever put in the effort to catch up you'll eventually be reviewing cards way past their due date and forgetting far too many of them.
You will be able to get 'max number of cards' on SRS in 4.0, though. (and all kinds of esoteric twists on that, like allowing cards that are a certain distance past due to not count towards that limit - basically you now build a list of of 'priority classes,' which can have various conditions / maximum numbers of cards associated with them, or you use one of our premade templates and don't think about that because there are only 4 or 5 options and we're doing everything else for you)
You will be able to get 'max number of cards' on SRS in 4.0, though. (and all kinds of esoteric twists on that, like allowing cards that are a certain distance past due to not count towards that limit - basically you now build a list of of 'priority classes,' which can have various conditions / maximum numbers of cards associated with them, or you use one of our premade templates and don't think about that because there are only 4 or 5 options and we're doing everything else for you)