How add a field with sequential numbers to a form

What is the point of the sequence number?  If you sort the recordset, would the sequence number now refer to a different row?  You can do something using DCount("*", "PK < " & PK) +1 which will count the number of rows with primary key values (autonumber is assumed here0 less than that of the current record.  If the data is grouped some way, the where argument of the DCount() would need to include the grouping field to count over the group.  This count would work ONLY if the rows were sorted in PK order.

This is quite easy to do with a report because the recordset isn't "live" and can't be sorted without rerunning the entire report.

The purpose is just to show the records as 1, 2, 3, etc.  No relationship to the PK.  So DCount("*", "PK < " & PK) +1 won't work.  The PK numbers are not sequential.

Unless you can utilize the actual sort sequence AND it is UNIQUE, then you won't be able to assign a sequential number.  How are the rows sorted?  Try concatenating the sort fields and using that to determine the count of earlier records.

the key is "that just shows a number that is not bound to the record source."

CAN you modify the structure? If so you could add:
TempRecNum, Long Integer, Default Value =Null

This temporary field could be updated as necessary.

While there is a way to use code to add a number to show as a "record number", it could recalculate and throw values off as the user moves if there is no way to bind it to values in the table.

Gustav often has great solutions -- this, however, was already suggested by Pat.  But he gave code with usage examples  ...

bottom line is this:
if there is a sort key that is unique, code can be used that ties the sort key to values in the table.