32 decoders open failed in one process

When we suppose to open 32 decoders through /dev/nvhost-nvdec node in ONE process, there alwasy failed after 16 have successfully opened, with following trace:
libv4l2: error attempting to open more then 16 video devices
[ERROR] (NvV4l2Element.cpp:63) Could not open device ‘/dev/nvhost-nvdec’: Device or resource busy

While if we open 32 decoders in more than ONE process, there will be OK, so could somebody please tell me why?

Please refer to