It may be an Autonumber field type which is a Long Integer data type, but be
aware it is not a number you can control and it may not always be incremented
without gaps. Autonumber fields should really only be used for primary keys
which are then used for relating records in master/child tables. The should
have no meaning in an application. If you need a serial numbering system
that will have meaning to the user or business, then you need to establish
your own numbering system to meet the business requirements.
If you need help with this, post back with the business rules of the
numbering system and I am sure we can assist with that.