Ah, I just assumed it was 3 in S1 (as mentioned in another comment) and 1 in S1 (pulled out of my rear). If it's 6 per vehicle, that would mean at least 540 fills, not counting scrubs and R&D tests.

For the broken link, there's just a stray semicolon at the end. Working version: http://spaceflight101.com/falcon-9-amos-6/wp-content/uploads...

Link not working, FYI.