In order to change the number of files BusinessObjects server is going to keep before deleting old trace files we can increase the value of keep_num variable to a higher number in BO_trace.ini. Alsoe, you can modify the entry to -1, which will not delete any logging files which are created and all the historical logs will be present in logging directory for reference.
Default location of BO_trace.ini file is mentioned below:
<Installation directory>\SAP BusinessObjects Enterprise XI 4.0\conf
Eg. <C:\Program Files (x86)\SAP BusinessObjects\SAP BusinessObjects Enterprise XI 4.0\conf