This PHP variable defines the maximum number of archives of each of SPIP’s log files (mysql.log, spip.log, prive_spip.log, etc..) which are stored in the directory specified by _NOM_TEMPORAIRES_INACCESSIBLES.
In fact, once any one of the log files reaches its maximum size, it is automatically saved under another name and a new empty log file is created.
$nombre_de_logs equals 4 by default (which actually corresponds to one file currently being written and 3 archived log files).
Application: if you wish to no longer have any log files, just add the following line to your config/mes_options.php file:
$GLOBALS['nombre_de_logs'] = 0;