35 million at best. Its pretty save to say that.
The Nvidia Tegra k1 and even the x1 (which isnt evend designed for mobile usage) both are already was inferior to the market leading ships made by Apple, Samsung and Snapdragon. Just check those benchmarks.
So EVEN if the Switch concept will work, Android and Apple will adopt very very soon and will deliver similar solutions (controller addons) to their smartphones and tablets. So people will be able to play skyrim on an iPhone as well (third party obviosly will also port those games to that huge install bases when they are doing a arm port for the switch anyway and its beeing succesfull).
So in the end people will just use their phones which are often as capable as the switch. In some years even WAY more capable.
Only reason to get the switch will be nintendos first party games. Only reason to get the wii u was that as well. We all know how that turned out.
I predict 25 mio units sold Lifetime.