Hybridization - Mixing one atomic s orbital with 2p orbitals
When some number of atomic orbitals are mixed together to produce hybrid atomic  orbitals, the same number of hybrid atomic orbitals must result.  So, when N atomic orbitals are mixed together, N hybrid atomic orbitals are produced.  In the case you describe, mixing one s atomic orbital with two p atomic orbitals means that three hybrid atomic orbitals must result.

These three hybrid orbitals will each contain 33.333% s character and 66.666% p character because that is the ratio of the one s orbital to two p orbitals that we are mixing together.  We can describe such an orbital as $\ce{s^{33.333}p^{66.666}}$, but if we reduce that notation to the lowest common denominator we get $\ce{s^1p^2}$, or the commonly used description $\ce{sp^2}$.  This simply means that this hybrid orbital is composed of one part s character to two parts p character.

To summarize if we mix 3 atomic orbitals, one s and two p orbitals, then 3 $\ce{sp^2}$ atomic hybrid orbitals will result.