This check confirms that the size of your database connection pool is adequate for Confluence. 

To resolve this issue

To resolve this issue you should increase your database connection pool. You will need to make sure your database can support the number of connections. 

Change the size of the database connection pool

 To understand if you are using JDBC connection(usually the most common) or datasource connection, you may want to review Check your database connection once and confirm before proceeding. 

If you're using a JDBC connection (this is the most common configuration) you will need to:

1. Stop Confluence (if Confluence is running).
2. Edit <home-directory>/confluence.cfg.xml and change the value of hibernate.c3p0.max_size and hibernate.hikari.maximumPoolSize  (if present). 
3. Restart Confluence. 

If you're using a datasource connection, you'll make this change in your datasource:

1. Stop Confluence (if Confluence is running).
2. Edit <install-directory>/conf/server.xml  and change the value of MaxTotal. 
3. Restart Confluence.

To find out more about configuring your database connection pool in a datasource see the JNDI Datasource HOW-TO in the Apache documentation. 

It may also be necessary to increase the number of connections allowed by your database. This is usually only required for PostgreSQL and MySQL. SQL Server and Oracle usually have sufficiently high limits.

mysql -u confluence -h localhost -p -e "SHOW VARIABLES LIKE '%max_connections%';"

psql -U postgres -c 'show max_connections;'

What will happen if I dismiss this warning? 

Confluence may slow down or time out during periods of heavy load because the number of connections configured for the database connection pool is too small to handle the number of requests.  See Confluence slows and times out during periods of high load due to database connection pool for more information.