I don't see that the number of subjects affect the dof ... assuming they are all in the same plane. If they are in a line away from you then your dof needs to cover the spread from the front to the back. You dof is dependant on four things:-
- your camera sensor size
- the focal length of your lens
- the aperture
- the camera to subject distance
Have a look at
http://www.dofmaster.com/dofjs.html Also the amount of blur of the background depends on how far behind your subject(s) it is - the further behind, the more blurred.
The rule you are referring to is nothing to do with portraits per se but its a way of estimating exposures out of doors without a light meter and of course ISO setting & shutter speed come into play here - have a look at
http://en.wikipedia.org/wiki/Sunny_16_rule With the inbuilt meter in your camera & using one of the metering programs (spot, centre weighted, pattern etc) in conjunction with exposure compensation if necessary should give you far more accurate results
