The backups are incremental, but it also depends on how much free hard drive space you had available when you create a new backup. If you have room to keep them all, then the older backups will still be there. If you are running low on space when creating a new backup, then Windows will automatically delete the oldest backup or two to make space available for the new one.
Anything deleted since the previous backup will not be included in the newer backup afterwards, but may be available to select to restore from an older incremental backup.
Hope this helps,