According to the official website, an extra episode of Rio: Rainbow Gate! will be bundled with the seventh volume of the Blu-ray and the DVD. It will be released on September 21st. The TV series is 13 episodes long.
Source: Rio official website
ref. length of other anime
Gosick: 24 episodes
IS: Infinite Stratos: 12 episodes
Mahou Shoujo Madoka★Magica: 12 episodes
Sources: Neowing, Neowing, Animate
Rio: Rainbow Gate Special on MAL
