They've only "ordered" 6. Apparently the Govt did a deal that the Saudis would take ships 5 and 6, with the UK taking 7 and 8. That way the yard would still get the work, and we'd have a bit more time to save some money in the piggy bank to pay for the last two.
We don't just need 8, remember we were initially supposed to be getting 12 of them!
We really need the 12 of them plus 18 Frgates if we get the 2 new Carriers. It needs 3 ships to keep one at sea constantly ( 1 in refit , 1 on passage to or from patrol and 1 on patrol / escort duties ) so with 6 that would leave us with 1 to escort any carrier and 1 for other Air Defence duties elsewhere in the world so not enough.