If you are backing up to Acronis Secure Zone, it will automatically delete the oldest files when there is insufficient space in that partition to accommodate a new backup. This only happens with a full backup. If there is not enough space to take an increment it will tell you either to increase the size of the Secure Zone or to make a full backup - it will then delete the oldest.
I don't bother with incremental backups now. An incremental takes almost as long as a full so I opt for a full backup every time.