The Maximum IMAP Connections Per IP Address is the number of IMAP connections that a single IP address can make at one time. How would you be able to increase the value?
You can increase the value in the WHM Home »Service Configuration »Mailserver Configuration.
Default Value = 20
The values you should configure all depend on how many active email clients are accessing IMAP.
We suggest that you start with the default values and then increase them if you start to notice the email activity warrants increasing these limits.
More details can be used for reference; https://docs.cpanel.net/whm/service-configuration/mailserver-configuration/.