There is theoretically no limit to the number of VoxWavs you can have running at the same time.
However, if you send audio from 2 or more VoxWavs at the same time they seem to interfere with each other, and it looks like only one of them will be understood (I am not sure if it is the first or last). Subsequent commands will continue to work for all instances, they only interfere with each other while both are transmitting audio at the same time, but then work fine afterwards so you can repeat your command and it will work.
If I get a chance I will look into this to see if there is a solution but I have a lot of other things I am working on so it may be a while.