According to Android Debug Bridge :
The server then sets up connections to all running emulator/device instances. It locates emulator/device instances by scanning odd-numbered ports in the range 5555 to 5585, the range used by emulators/devices. Where the server finds an adb daemon, it sets up a connection to that port. Note that each emulator/device instance acquires a pair of sequential ports — an even-numbered port for console connections and an odd-numbered port for adb connections. For example:
Emulator 1, console: 5554
Emulator 1, adb: 5555
Emulator 2, console: 5556
Emulator 2, adb: 5557 ...
The ADB server only checks for devices by scanning odd-numbered ports in the range 5555 to 5585 ( 30 ports in total) and assigns 2 ports for each device. Is ADB capable of accepting more than 15 Android Devices (15x2 ports) or can I connect more devices on the same computer? I think it is impossible to run more than one ADB servers on the same machine.
You can run more than one instance of adb on the same computer using the environment variable ANDROID_ADB_SERVER_PORT. see more details on this answer.
With the -ports option on the emulator, you can specify any port, and then do:
$ adb connect localhost:PORT
If your phone is rooted you can restart adbd on another port (see another answer), and then:
$ adb connect IP_OF_DEVICE:PORT
The device is then available over wifi and does not need to be plugged in. Be aware that anyone on your wifi network can access your phone this way!