Neat little tidbit:
Apple’s total number of configurations for mobile devices (iPod, iPhone, iPad) has nearly tripled since 2007. iPods have traditionally been the majority of these SKUs (Apple still sells 7 colours of iPod nano and 5 of iPod shuffle). Since 2011, though, the total iPad/iPhone configurations have drawn about equal to the iPod at 25-27 apiece, primarily from Apple keeping old hardware versions around as the “low end” options.
It might also be interesting that the average number of units sold (annually) by configuration option has stayed fairly consistent; around 4,500,000/ configuration for iPhone/ iPad, and 2,000 for each iPod.
You can pick up the spreadsheets showing Apple's mobile device configurations by year on the brand-spanking new Downloads Page.