From the Negi no Ochakai event held today, Akamatsu Ken mentions that if preorders for “Mahou Sensei Negima! ~Shiroki Tsubasa Ala Alba~,” which ships along with Volume 23 of the manga, exceeds 50,000, the staff will consider a film adaptation. 100,000 sales will instead convince them to produce a 3rd TV series.

The preorder-sales counter on the official website, which counts volumes 23-25, currently stands at just over 10,000.

[Source]