During any time except prime time, you have 3 tuners that can be used to view live content. If I am not mistaken, during the viewing of those three channels, you could also watch something off of the DVR or on demand.
During "prime time" if you enable Prime Time Anytime, then all four of the major broadcast networks (CBS, ABC, NBC, FOX), if you have them in HD, will record on one tuner, leaving two open tuners to watch other shows. This effectively gives you 6 programs to watch at any one time during prime time, assuming you watch the main networks. As well as the DVR content and on demand that I mentioned before.
If you do not enable the "PTAT" feature, then the first part that I explained would apply at all times.
If I am wrong, then someone can come and correct it. I am fairly new to the hopper joey setup.
Edit: Using sling would take up one of the available tuners whenever it is active.