I see. I didn't keep Paul's reply, so apologies if I'm parroting him,
but ...

If column  D will be empty unless it has the sequence  number in it, then you
can use an anchored "max" call. E.g. my active range  is D2 to Dnnn. In D8
I have =MAX(D$2:D7)+1. D2:D7 are empty. Result: 1. Copy to D19, where it
reads: =MAX(D$2:D18)+1. Result: 2. Copy to D35, where it reads:
=MAX(D$2:D34)+1. Result: 3.

Copy it to D13, where it reads: =MAX(D$2:D12)+1. Result: 2. D19 now 3; D35
now 4.