Every time that you sync, iTunes will make a backup first in case something goes wrong or you change your mind. But there is only one backup saved at any time and that is the most current one, thus there is no space wasted. Any previous backup is overwritten.
Wasn't there a time when backups were not overwritten and new ones were made for each sync? I remember reading something like that somewhere.
Yes. My 3Gs (2009) allowed for multiple backups which I always renamed to the date created. When iPhone 5 came along, the option had disappeared. (I don't recall whether or not my iPhone 4 still had that option.)