More than the Wii U, less than the 3DS.
The rest depends on the price development, software support and future revisions.
If they manage to shrink the device to Vita/3DSXL size & weight, enhance the battery life and lower the price in 2 or 3 years (as an additional option to the bigger model), they could win back some handheld fans who don't want a clunky hybrid, it could exceed 3DS numbers.