You aren't going to get a figure. Xenforo Ltd themselves don't actually know. The best they could do to generate a figure is one looking at expired licenses to see the latest version available to all given accounts which would not include people not actively running a site. Then you counter it with downloads for active licenses which does not mean its installed at that version yet.
Most of the sites still on older versions that are not on an expired license would be places like IGN which have significant core changes into their 1.3 build. As well as anyone invested in add-ons that cant make it to 1.5.
1.2 was a theme breaker for some sites but beyond that ive only seen a couple of addon's truly screwed between 1.2 and 1.5 that matter to some people enough to stay behind. If you are starting out I would just shoot to 1.5. In the end its where most of your support will be.