How many Android OS have been released yet?

Android mobile operating system began with the release of the Android beta in November 2007.

Android is under ongoing development by Google and the Open Handset Alliance (OHA), and has seen a number of updates to its base operating system since its initial release.

There are 10 android os (Jan, 2014) released yet, they are named alphabetically.

Android 1.0  (First commercial release, 2008)
Cupcake (v1.5),
Donut (v1.6),
Eclair (v2.0–v2.1),
Froyo (v2.2–v2.2.3),
Gingerbread (v2.3–v2.3.7),
Honeycomb (v3.0–v3.2.6),
Ice Cream Sandwich (v4.0–v4.0.4),
Jelly Bean (v4.1–v4.3),
KitKat (v4.4)

There are rumours that next Android OS, means Android 5.0 will be named as Keylime-pie. Release date has not been announced yet.