Paintchat (shipainter pro) that you install onto a website (You can try getting on one RPM if you wanted to) can support up to...uh, as much as you want until the server crashes? Of course the admin can set a limit. I've been in paintchats with like 20 people before, but only 5 of them were drawing though.
I used to go around oekaki and pchat sites a lot, but there seriously isn't any 'perfect paintchat' program. There is one in Pixiv though, but I don't think that's been translated yet. Also I have no idea how to use it because I never tried but I have snuck in and watched a few of my favourite artists duking it out it was fun. The actual tools/app for Pixiv look pretty good from what I've seen people make out of it but then there are always oekaki genies who can apparently make amazing stuffs from the most simple of apps.
Oh and Koi:
[Ctrl][+ ] / [F5] "ZoomIn"
[Ctrl][-] / [F6] "ZoomOut"
[SPACE][Ctrl][Click] "ZoomIn"
[SPACE][Ctrl][Alt][Click] "ZoomOut"
[Ctrl][Alt][+ ] "ZoomIn (Auto window resize)"
[Ctrl][Alt][-] "ZoomOut (Auto window resize)"
[Ctrl][Z] "Undo (Max 8 times /layer)"
[Delete] "Clear layer"
[Insert] "Fill layer use brush color"
[SPACE][Drag] "Scroll view"
[Alt][Drag] "Rotate view ([Alt] to return)"
[Ctrl][Alt][Drag] "Change pen width"
[Right click] or [Shift] "Spoit: Pick up cursor position's color"
[Ctrl+Spoit] "Pick up ActiveLayer's color
[Tab] "Show/Hide, ToolWindow"
[A] "BrushTool/ Pen"
[S ] "BrushTool/ Watercolor"
[D] "BrushTool/ Blur"
[F] "BrushTool/ Eraser"
[E] "BrushTool/ Change to right setting"
[R] "BrushTool/ Change to left setting"
[Change to Blur with double click] "Former blur brush"
[Ctrl+ Layer visible button] "Set the layer visible inverse"
I can organise stuffs like, now, if you want? >_> Seriously we just need to all be at the same place at the same time and I can poke the rest of you \o/