If there are two values in the field, which one determines the sort order? The first one? The smallest value? The largest?
The first one. The entry may be: "12, 13" or "12, 300" but having it partially in order would make inventory much easier. So ideally these entries would fall between 11 and 14.
Define a calculation field and sort on it. Define it like this and select "number" as its return type:
Getvalue ( Substitute ( Tank ; "," ; ¶ ) ; 1 )
This expression will extract any number to the left of the comma.