Maximum number of real devices

Hello Appium folks,
First thank you all for this great tool, and all the information shared here. With all this help i managed to build a platform to automate 15 Android devices on different versions (4.2.2 to 5.1.1), using Appium as nodes of a grid, controlled from Python client.
When i add the 16th device, it seems that i hit some limit. The message i get is:

{ selenium.common.exceptions.WebDriverException: Message: A new session could not be created. (Original error: UiAutomator quit before it successfully launched) }

The other symptom is that adb loses the list of devices.

Any idea what the bottleneck is? and how to go beyond 15 devices?

Thanks in advance

adb server limit 30 prot for devices,1 devices occupy 2 port