Destiny isn't following a "1-10" numbering system. It's going to be like World of Warcraft.
Watch the 30 minute interview, you'll see how they talk about Pre-TTK and actually REFER to it as Destiny 1, and then how they're glad to be now able to bring the "fans" the second installment.
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There are going to be 5 different destinys because they have a 10 year contract and to every game there is 4 expansions and a comet, the comet goes to the 1 year anniversary to each destiny.