The balanced equation is:
\[
\mathrm{Ca(OH)}_2 + \left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4 \rightarrow \mathrm{CaSO}_4 + 2\mathrm{NH}_3 + 2\mathrm{H}_2 \mathrm{O}
\]
The sum of all coefficients is:
\[
1 + 1 + 1 + 2 + 2 = 7
\]
\[
\boxed{7}
\]
The answer is (B) 7.