BUG #16232: Database server connection limit exceeding

The following bug has been logged on the website:

Bug reference:      16232
Logged by:          DEEPAK RAI B
Email address:      deepakraib@gmail.com
PostgreSQL version: 9.6.9
Operating system:   Core OS
Description:

Hi Team,

We are getting below error even though total connection in the database
(select count(*) from pg_stat_activity;) is not more than 10 and
max_connections are allowed 300. We are seeing below error most of the time
and only sometime it is impacting our application. Please let me know what
might be the issue?  

"startup",2020-01-23 15:17:25 UTC,1999/7,0,FATAL,53300,"remaining connection
slots are reserved for non-replication superuser connections",,,,,,,,,""