We don't have an exact number yet, but we're designing the game for team play, and we will definitely support more players per game than Starcraft did. GameFan said 12 players per game. Here Blizzard doesn't confirm this, only stating that there will be more than 8. We're using non-proprietary file formats for everything in the game, because we want to make it very easy for people to customize the game and do total conversions. Our geometry, animations, and unit stats are stored as text files.
Sound is in wave files.
Textures can be pcx, tga, bmp, etc. Script files ship in source code format. This has been previously known. Some very interesting things are going to come out of it. While we have not yet discussed what we will be doing as far as getting ongoing information about the game out to our fans will be, but we do intend to support the Warcraft community.