OVER NINE THOUSAND.
Around 14 or 15 looking at the achievments, cant recall exactly how many though.
"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones."- Albert Einstein.
You think you've got l
14, but you don't get to play 12 and 13. (DLC?)
There are technically 11, as 11-13 are corrupted and unplayable. You don't miss anything storywise, though, except the impression Ezio and company have been going on adventures in the meantime. From what I've seen, Discovery fills in the blanks.
Seems fairly likely that Sequences 12 & 13 will be DLC. The southern area of Florence across the Arno and the island of Giudecca in Venice look like they will probably be unlock-able. There are 11 years and 7 memory slots between Sequences 11 and 14, so I think they have something up their sleeve.