The mass number of ending nuclei reduced to $20 \mathrm{units}$, 

One ${}_2{}^4\mathrm{\alpha }-$particle has $2+\mathrm{ve}$ units of charge and $4 \mathrm{units}$ of mass, therefore if w subtract the mass number of product nuclei from reactant nuclei we get $20 \mathrm{units}$ of mass, which can only be compensated by $5\mathrm{\alpha }-$particle.

${}_{88}{}^{226}\mathrm{Ra}\to {}_{78}{}^{206}\mathrm{X}+5{}_{10}{}^{20}\mathrm{\alpha }\to {}_{78}{}^{206}\mathrm{X}+4{\mathrm{\beta }}^{-}\to {}_{82}{}^{206}\mathrm{Pb}$

Thus, $5 \mathrm{\alpha }-$particles emitted during the radioactive decay.