There is technically no limit BUT:
A good story structure involves a certain process and a number of related functions (related to archetypes). Normally, you keep the story clean by assigning multiple functions to a small group of characters. Once you have the story outlined well, you can then distribute the functions among a number of characters (if you so wish).
I think you're probably going awry by assigning each function to a character. Try and assign multiple functions to a character.
It might work on a large canvas of a novel etc, but in screenplay terms, you're more likely to confuse than not.
Also, think about groups of people as a single set. The group is the archetype. A group / set means that you can pick out different individuals from the group to perform the function. This may help if you really want lots of characters.
Again, it's probably heresy to say on this board, but a good source of info for groups of characters is Kal at http://www.clickok.co.uk/index4.html
; for example, he groups a lot of characters into sets such as "The Good People of the Township" and "Circle of Elders" etc and it makes a lot of sense. Email him; I've done so and he's quite helpful.