I can only present a low-case, expected, and high-case scenario:

Low case: Switch fails to turn up at E3, with few third-parties actually on board with anything of substance. Rumours about how third-parties did not expect the initial buzz go around, but ultimately third parties decide to treat the Switch just like the Wii -- resulting in low-quality past-gen ports, quick mini-game and handheld-focused simple projects. Mario Odyssey is delayed to 2018 "to ensure optimal quality". Without a big new exclusive, Switch loses steam as people remember the sting of Wii's final 3 years. Sales drop to levels similar to Wii U's first year.

Sales: 8.7 million

Normal case: Good software line-up based on exclusives by both Nintendo and its second parties as well as select third parties. A low number of high quality 3D ports from current-gen consoles. A number of good ports from old-gen systems (both by Nintendo as well as by third/second parties). The system retains its current cult status into the winter, and is further boosted by the excellent Mario Odyssey. The system remains in relative scarcity during the whole year, and stocks are never in full availability.

Sales: 12.8 million

Best case: An excellent demonstration of support by third parties, alongside meaningful Nintendo first and second party games shows strong intent to keep the Switch focused on 'quality games', rather than quick cash-grabs. Mario Odyssey goes on to exceed Breath of the Wild in review rankings, and sells very strongly during December, raising the system's appeal. The system is very hard to find throughout the year.

Sales: 14.3 million

Green098 said:

I'd rather wait until E3 so I can see the Switch's full first year line-up, but at the end of day I think there is one game that will make the biggest difference in the Switch's first year sales.

14 million+ with Pokémon

Around 10 million without