while i think more than one backup is generally excessive, it depends on the person and the product.
if you don't have an insane mac addiction with backups and LEs you have never touched flying out the rear, and a lipstick or eyeshadow comes along that is your DREAM colour, your HG, something that flatters you like nothing before... get an extra or two. or if you know you go through lipglass in a month and you really love a unique shade, grab a couple. same with eyeliner or lip liner, as i go through the former very quickly.
chances are, by the time you go through your original product, mac will have released something that is similar enough, or maybe even rereleased it.
i sold and am trying to sell most of my msf backups, knowing i will never ever ever use them. by the time i use the main one up, i will be so over the colour or find a better product.
what i dont get is backups of perm colours