The 3DR and xbee modules are spread spectrum, and support a netid. There are up to 253 useful netid's, and the 3DR supports programmable frequency spreads and channel numbers.
If you do not change your settings, you may be broadcasting illegally for your area.
You may also be broadcasting on the default netid that ships (on either 3DR or xbee) in which case the answer is one.
Otherwise, the answer is at least serveral dozen, if they all have different netid's, with better results for widely spaced flyers using the required minimum number of channels in the 3DR.