Now that I've played I know why the core set only handles 3 pc'c. So the question is, how many copies of each basic action card will be in the Adventure's Toolkit.

There's an official answer to this question in the following thread in the sub-forum "Rules Questions":

www.fantasyflightgames.com/edge_foros_discusion.asp

In short, it has enough basic Action cards to fully support a 4th PC. By fully I mean without any sharing at all.

Although apparently a great deal of the cards can be shared easily- but many cards aren't available to all players due to certain stat requirements, so sharing may even be irrelevant.

In essence, each copy of the toolkit provides enough materials for an additional player, plus more careers and stuff.

Yeah what he said ^ ^ ^ .