The Sylow Theorems · Easy problem

The dihedral group $D_{11}$ (the $22$ symmetries of a regular $11$-gon) has order $22 = 2\cdot11$. Each of its $11$ reflections generates its own subgroup of order $2$, and these are the only subgroups of order $2$. How many Sylow $2$-subgroups does $D_{11}$ have?