This page leads to multi-touch implementations with TouchDesigner.
See TUIO to roll-your-own.
See iOS and OSC for various ways to get multi-touch data into from the Apple iOS devices.
A touch on a panel will as as a left mouse button event. This lets you used multiple panels such as buttons, sliders via multi-touch as they are. If a panel needs to handle more than one touch, you can disable the built in multi-touch behaviour via Panel COMP Panel Page's Multi-Touch parameter, and then process the incoming touches to that panel via the Multi Touch In DAT.
The first touch in the Network Editor will act as a left mouse button event. Subsequent touches are ignored.
The number of touches available is limited by hardware.
Windows 7/8 only allows the use of one touch screen at a time.