Donald, how many of the Provinces, Estates and Curses will there be? Enough for 5 or 6 players? Now that I'm thinking of it, are you supposed to have more than 12 Colonies if you play Prosperity with 5 or 6 players?
The base cards product only supports 2-4 players. Obv. you can add it to Dominion or Intrigue to support 5-6. Supporting 4 players took 249 cards, and these numbers have to be round; we did not think most people would be thrilled paying for more cards they didn't need.
The Prosperity rulebook indicates that you have 12 Colonies for 3 or more players. That includes 5-6 players.
Thank you Donald for your explanations.
My "problem" here is that I usually run "mid-size" tournaments in my house. I had all expansions on the market so far and I hat to play in more than 3 players (ok, I hate 3-players matches too).
My current setup, listed [num_players ___ comma separated players in each table, starting from "main table"]
3 (3)
4 (2,2)
5 (2,3)
6 (3,3 or 2,2,2 it depends how many noobs are in the tournament)
7 (2,2,3)
8 (2,3,3)
9 (3,3,3)
10+ sorry guys, it's impossible, too few cards

Problems:
- I can share low treasures (coppers, silvers and golds) among tables, relaxing the rule of empty piles over these piles (ok, with hinterlands traders and tunnel I may have some problems, not yet played in tournaments)
- I try to share Potions, I usually look at the kingdom cards and I give some potions here and there, if some table is running out of potions, I'll move froma table to another, or, in worst case, I use white cards
- I go somewhat crazy with low green cards... I had to produce for 3 tables 12 duchys, 12 provinces, something like 24 estates... I've printed them on papers and put the printed&cut cards in the sleeves of some unused kingdom card (imagine the setup of a tournament turn! I becoem skilled at it)
- I go utterly crazy with Colonies and Platinums... And we all love Colony Game
Conclusions:
- I can afford (and I will) another copy of the Base Dominion, but I don't want another Prosperity.
- I hope the "yet-to-come" Basic-Card-With-Art Expansion will contain more than 12 colony-platinum
Thank you for your time!