What is the common oxidation number of bromine in its compounds?

Usually $- I$, but not always.
As a halogen, bromine commonly exhibits a $- I$ oxidation state (i.e. valency) in its ionic compounds as the bromide anion, but it does not do so exclusively. Bromine does form so-called interhalogen compounds, e.g $B r {F}_{3}$, or even $B r F$ (and what is the formal oxidation state of $B r$ in these two examples; they are different in each case). Oxidation numbers of $+ I$, $+ I I I$, $+ V$, and $+ V I I$ are known with its oxides, and oxoanions.