Create a real number field with the benefits of filtering due the nature of the field.
Make sure your system meets the minimum requirements:
The number fieldtype works just like any other fieldtype, it also looks like a normal fieldtype but in fact it is not a real simple field. The number fieldtype convert in the MySQL database a field to a integer field. This way it can hold any number value like 9 or 100 or 2819.
Because the field is now converted to an integer, the filtering (orderby) in the {exp:channel:entries}
works perfectly
Because the number fieldtype convert the field in mysql to a integer field, it is wise to delete this field if you want to change it to an other fieldtype. Otherwise that field is an integer field and not a text field.