Mardon, I don't know explicitly how to determine the number of mtDNA full-sequences uploaded to GenBank by FTDNA users, but I tried two different searches, one based on your post, to see if I could find how many FMS uploads came from FTDNA data.
Search #1 was: mitochondrion "homo sapiens" ftdna "complete genome"
It returned 2,031 hits.
Search #2 was: mitochondrion "homo sapiens" greenspan "complete genome"
It returned 3,029 hits.
I don't know if either of them are correct. I'll ask Ian in the near future if there's a better way to search. But if I had to guess, I'd say that the 3,000 number probably is a realistic top-end so far for kits originating from FTDNA. Makes sense because GenBank has no relatedness matching service, and I would imagine that only a small minority of people buying an FMS test as a consumer ever bother looking into GenBank.