You can use GetNextNumber() function.
Syntax : GetNextNumber(record.fieldname,max_range);
e.g. : GetNextNumber(Emp_detail.emp_id,99999);

But it's better to get the maxium value of that particular field and increment it by +1. You can write code in page activate.

It would be better to set up auto numbering for that particular field.
SEtup>Common definition>Codes and Auto Numbering.

Now call the delivered function and assign the auto generated number to your field.

Regards,
Rahul