The following reasons affect the number of backups you'll want to keep:
The Backup Tool has two methods of maintaining backup files: Placing files being backed up to a single backup file and Creating a new file for every backup. If you don't need to keep more than the last copy, choose the first method. Otherwise you have to choose the second one.
Setting the 'Store the last' value, keep in mind that if you've set the project to back up only new and changed files, you may encounter the situation when the first full backup is removed on creating the partial backup number 'Store the last' + 1. To avoid it don't set this value to be very little and do full backups from time to time (say, weekly or monthly).
As for the most critical backups, consider the possibility to make several copies of the backup on other devices than the ones selected as a Storage Folder. Exporting a backup, the Backup Tool allows you to place several copies of the backup to one folder. It's especially useful when you have to use an unreliable media (like floppy disks).