The minimum number of full backup chains would be exactly 1 backup chain. So every time you create a new full backup it would delete the previous full backup chain. But by default we won't be deleting anything. Amazon storage is cheap enough that I don't think it makes sense to delete backup data for users unless they ask us to explicitly. I talked to Alon about the logic and we will be implementing this shortly.

PS: Sorry for the absurdly late reply. I didn't notice you posted a follow up question until now.