There were 7 built originally by the Forerunners. The Halo in the first Halo game is Installation 04, this one was destroyed. Halo 2 features Installation 05, this one is still intact. A replacement for the destroyed Installation 04 was constructed in Halo 3, but was destroyed as well (due to it firing before it was completed). So there are 6 installation still out there.
If you mean games, then 6. In release order: Halo: CE, Halo 2, Halo Wars, Halo 3: ODST, Halo Reach.
12 originally built but all but one destroyed by the flood during forerunner flood war then 6 more were built to replace the 11 that were lost the original 12 were bigger than the ones we've seen in games but smaller than the ark so one of the halos is bigger than all others but smaller than the ark (like I said) so I think this halo that is of the original 12 might have something to do in future games and books storyline (p.s. If you do not know about the upcoming games and books made by 343indutries then well start searching "halo 4 and upcoming halo books")