Error: [ldap] All ldap connections are in use

We have started seeing problems our radius server with the Error "Error: [ldap] All ldap connections are in use" We have increased the "ldap_connections_number" from 5 to 20 which has largely resolved the issue. we now receive over 100,000 authentications a day, is there any guide to "ldap_connections_number" for the number of authentications or should we just keep increasing the number until the issue goes away