Yup - minimum of two. If the fax/phone and Sky receiver have separate plugs you may need one each, thus three, but if they're already plugged into a double adapter, still just the one.
It's wierd that I'm writing all this bumf yet am still only on ISDN ! Still, 5th November and counting !
BTW, if anyone's interested in becoming a broadband campaigner, it's well worth it - I received £100 of "cheques" which I can exchange for vouchers to be spent at a wide variety of stores - PC world may get my money for a splendid new ADSL modem.