Something between 1.0 and 1.5 million. I´d bet 1.4 million. It could do more if Nintendo makes a good market / advertsiment campaign for it.
Are those sales numbers for Mario Tennis Open (3ds) accurate ? I couldn´t find the 2013/2014 sales report on the Nintendo website, but I was sure that it sold at least 1.1 million physically in its first year and some good numbers on digital.