If you enable the option "Generate Logs when Session Starts", what effect does this have on the number of traffic log messages generated for each session?

This option specifically enables session start logging, so traffic logs are indeed generated.

By default, one log message is generated at session end; this option adds a second log message at session start, making it two.

When 'Generate Logs when Session Starts' is enabled for a firewall policy, FortiGate generates one traffic log entry when the session initiates and a second entry when the session terminates, totaling two logs per session.

Traffic logging is distinct from security event logging, and traffic logs are generated for sessions as configured, regardless of security events.