Assuming that the form is not bound to the data source (table/query) then the field where the claim number resides would have the control source set to =DMAX([ClAIM_NUMBER], [Your_table_name_here]) + 1
Write the claim number when the user commits the record to the database.
WARNING: this causes huge problems if it is a multiuser database. For example what if a person starts entering a claim number when another is half way through entering there claim. You may want to assign the claim number at the time of commitment or may I suggest you have a system table which is accessed by a bit of code that retrieves the next free claimnumber from this table and then writes the next available number to this table. When this table is being accessed, ensure that the table is locked (unavailable to all other calls) by this code until it is finished.
If this has been helpful, please mark this post as helpful. I would appreciate it immensely!
D.