I have six players in my group and, while dice won't be a problem because two players have their own copies of the core box, I'm concerned that cards and their usage might be an issue.

The two players with their own copies of the core will be using the core material for their characters. This is so we don't end up having to keep track of every card and whose set it came from.

I have, or am waiting for delivery of, one of everything but the player's vault, GM's vault.

My main concerns at present are with condition cards, wound cards etc.

For those people who have run with a larger group size, what were the things you really needed more of? And is it better to get a player's vault+gm's vault (with updated errata but less dice) or another copy of the core rules.

I believe you don't need anything extra. I've four players and have never given out more than one condition card. What's even better if you've Omens of War you are more likely to stumble upon the "good stuff". I actually already removed over half of the wound cards as they just aren't necessary but mainly a pain in the ass.

Yep. You've got it covered.

If you ever do find yourself in a tight-materials pinch...flip a condition card face up so it can be read adequately by all concerned, and then have people use a post-it or some other self-record-keeping device to note that they are also suffering from Crippling Diarrhea. It's not the best or most efficient use of the game system but it will work. You can do the same for Action Cards, Talents, etc. And as the other two knowledgeable fellas above said, you probably won't have anything to worry about.

As long as no one throws up their hands and quits over having to write something down for a session...you're golden.

I run two groups of six, and I think you have it covered because dice is the issue. One of the things I do to speed up game play is roll all the negative dice and let the players focus on the positive dice.

Also, in the rare case where people choose the same card - it happens less than you would think - I run off a copy of put it in a card sleeve. Problem solved very quickly.

The bigger challenge is ensuring the group is all on the same page, but that's a roll play issue and not WFRP specific