You could have something like this in a text box's default value property:

Nz(DMax("FieldName", "tblTableName")) + 1

where "FieldName" is the name of the field and "tblTableName" is the name of
the table where "FieldName" is contained.  You wouldn't even need a button
if you used this method.  If you want it to start at 500 then the easiest
way would be to have a dummy record where the value is 499.

Incidentally, it's considered to be good practice by a lot of developers to
split a database where there will be more than one user.  Check the help for
"split database".

HTH - Keith.
www.keithwilby.com

In a multiuser environment, there are at least three ways to do this.  Which
way you choose will depend on how your form works and what you want to happen.

First way is to create the number when the user selects a new record.  Then
in the Before Update event of the form, check to make sure another user has
not already saved a record with the number.  If they have, then create a new
number or alert the user to take some action.

The second way is to wait until all other data in the record has been
entered and create the number in the After Update event.  In reality, it is
still possible that two users could collide, so the check for a new number is
necessary.

The other way is to get the next number and immediatley create the record
before any data entry starts.  This way is the least likely to get a
collision; however, the danger is that if a user cancels the update you will
either have a record that is empty except for the number or, if you delete
the record, a gap in numbering.

Now, I see you had another post with a way to increment the number.  I think
I would do something like this:

Me.NextNumber = Nz(DMax("[MyCounterField]", "MyTable"), 499)  + 1

This way, you don't need a dummy record.  The very first record created will
get a null value from the DMax.  The Nz will return 499 and you will add 1 to
that creating 500 as the first number.