yup....don't stop.... serioulsly though just take biiig breaths throug your nose inbetween the rounds, and try and exhale when punching or getting punched(to avoid getting winded)
12-20/minute is optimal unless you're really suckin wind and exhausted. try to keep your mouth shut so your jaw doesnt get cracked in half.
Between rounds, slow your breaths down as previously stated, by drawing slow big breaths to increase the oxygen intake.