The number of terminal oxygen atoms present in the product B obtained from the following reaction is ________.
$\mathrm{FeCr}_2 \mathrm{O}_4+\mathrm{Na}_2 \mathrm{CO}_3+\mathrm{O}_2 \rightarrow \mathrm{~A}+\mathrm{Fe}_2 \mathrm{O}_3+\mathrm{CO}_2$
$\mathrm{A}+\mathrm{H}^{+} \rightarrow \mathrm{B}+\mathrm{H}_2 \mathrm{O}+\mathrm{Na}^{+}$