You need to number with the leading zeros eg. 01, 02, 03... 09, 10, 11 or with two leading zeros if 100+ songs e.g.. 001, 002,...009, 010 and so on. However if you use the "track number" field instead of "composer", rekordbox will order it correctly without the leading zeros as it appears to treat this field numerically instead of alphabetically. If you don't want to use the 'track number' field for this purpose, try a numerical field such as 'disc number', this should fix the issue.