When a person is baptized they are added to the number of 8,000,000.
When a person is DFd does the Org subtract them from that same number or not?
The "8 million" is the count of people who turn in a "field service report", i.e., the number of "publishers".
You don't have to be baptized to be a publisher (although the vast majority of JW publishers are baptized). Most unbaptized publishers are kids of JW parents.
In order to be baptized as a JW, you have to have been a publisher for many months beforehand. It is a set-in-stone requirement, no exceptions.
So when someone is dunked, nothing really changes - he was a publisher the month before his baptism, and he continues to be a publisher after baptism. The count does not change. The change occurred many months earlier, when he turned in his first field service report.
When someone is DF'ed he stops turning in field service reports. He is no longer a publisher. So yes, when DF'ed, the count goes down.
The count also goes down every time someone becomes "inactive" (stops turning in field service reports), and (obviously) when a publisher dies. Based on best-guess estimates, there are probably around 80,000 JW deaths every year, mostly (like everyone else) due to natural causes.