980 connection limit

I used emqtt_bench to stress test my EMQX Server, but found a strange phenomenon.
After rebooting EC2, I used emqtt_bench conn every time for 1,000 transactions, and there was no problem several times.
But as long as I restart EMQX, the number of connections can only reach 980 (the same in every test), and what happens next
“waiting for connack” error message (as shown below), does anyone know about this situation?


Please make sure the number of available file descriptors is >>1024 on both loadgen and EMQX side: Performance Tuning (Linux) | EMQX Documentation