It sounds to me like Dave was engaging in a common form of understatement, such as, "Oh, I think I've got a few more good years left in me." The words "at least" are implied in the usage.
Vanguard's an open-ended series; there's no carved-in-stone plan for how long it will continue. It can and will last as long as the sales figures justify its continuation. So we don't know in advance whether those sales will stay strong enough to keep doing it, so it's reasonable to say "I hope there will be at least a couple more books." But by the same token, nobody in or out of Pocket has any idea when the series will end, so there's no reason to assume that "a couple" is all there will be. I expect Pocket will keep publishing them as long as you guys keep buying them.
