I just wanted to point out that the pattern does not say to cast on 12 sts, but that it is a "multiple" of 12 sts. So I would think that you follow the pattern as stated (e.g. K2, P2, K2, work 6 sts in garter) and repeat however many times needed across the row.
"Be afraid of bears, of bungee-jumping, of faulty wiring in old houses, but never, ever be afraid of trying something in knitting." Stephanie Pearl-McPhee, Knitting Rules
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