Log contains last 100 lines by default, as it's stored in memory (and it's cleared upon reboot). To change this limit, or to change log to "disk" you have to configure it in the "system -> logging" menu.
First, choose where you will store specific logs, for example if you store INFO logs in memory, open the MEMORY action and change the lines from 100 to anything else you want:
Of course, you can change the INFO logs to be saved to DISK (if your disk is magnetic, not NAND flash, because NAND could be damaged if you have thousands of log lines every hour). Then open the DISK action, and also increase the stored lines number, and set how many log files you will keep.