The OVA series contains 4 episodes.
The new OVA series Shakugan no Shana S Vol. 1 will be released on 23 Oct in BD/DVD format. It is about a short story in Itou Noidi illustration collection Gu-ren, which is Reshuffle.
Vol. II : February 2010
Vol. III: May 2010
Vol. IV: August 2010
If I am not mistaken, I heard that there will be Shakugan no Shana-tan Revenge and a special video of new OVA series as DVD appendix for the Shakugan no Shana II Official Guide Book.
As such, hopefully, there will be Shakugan no Shana Season 3 during fall 2010.