Nisemonogatari is 11 Episodes Long

According to Otona Anime magazine vol.23, Nisemonogatari was announced to be 11 episodes long. Director Shinbo Akiyuki intended to reproduce the high pace of the conversation of the characters in the novel with the relatively short length of the series. Shinbo also said he wanted to animate all the Monogatari novels and catch up with the newly published Koimonogatari. There have been twelve Monogatari novels published so far and the whole series will be concluded by the next three volumes: Tsukimonogatari, Owarimonogatari, and Zoku Owarimonogatari, to be released this year.

Source: Scan of Otona Anime