Toshiba cordless devices have a range of 6 feet, but under certain conditions, they can exceed that. To avoid possible interference, try to keep the distance from system to system at least 10 feet. Also, moving the mouse closer or further away to the receiver can also improve transmission and performance.
It is also recommended that each cordless device be installed in half hour increments. This is because when the receiver is first plugged it, it is looking for devices that will be synchronized to it. Even after synchronizing your device to the receiver, it still "looks" for other devices for 25 minutes. If you install another cordless product at the same time in close proximity, it is possible when you press the connect button on the second cordless device that it synchronizes to the receiver for the first computer.
There aren´t a specific number of cordless devices that you can/cannot use in one area, as there are a lot of environmental variables that will affect the performance of the cordless device. It is feasible to use many units in one area, provided you install them incrementally and be aware of the environmental variables discussed in this document.