Multitouch is supported, with two caveats:
1) The Touch.X and Touch.Y expressions only return one value, so only can ever return one touch
2) Android's browser doesn't properly support multitouch, but it should be fixed this month with the 'Ice Cream Sandwich' (Android 4) update. iOS should work fine though.
If you use sprites on-screen as touch buttons, then use the 'is touching object' or 'on object touched' events, multitouch should work fine (once Android 4 is out).