Parameter and Command Reference > Logging Configuration Parameters > LogMaxOldFiles 

LogMaxOldFiles 
Maximum number of log files in the log directory. The maximum number of log files the specified LogDirectory can store before the application runs the specified LogOldAction. If you do not want to restrict how many log files the LogDirectory can store, type -1.
 
Logging and/or LogStream or TaskName
If you set this parameter in the Logging and LogStream sections, the setting in the LogStream section takes precedence for the specified log stream.