Is there a maximum number of AirPlay receivers that I can output to at the same time?
The maximum number of Airplay receivers are 3 to 6 as noted in iTunes: About playing music with AirPlay


  The maximum number of outputs are three to six in typical conditions. The number that works for you will depend on your environmental conditions (such as building composition and local radio interference), distance to the outputs, and available network capacity relative to your other usage.