$\begin{aligned}
& 50 \mathrm{VSD}=49 \mathrm{MSD} \
& 1 \mathrm{VSD}=\frac{49}{50} \mathrm{MSD} \
& \mathrm{LC}=1 \mathrm{MSD}-1 \mathrm{VSD} \
& \mathrm{LC}=1 \mathrm{MSD} \cdot \frac{49}{50} \mathrm{MSD}=\frac{\mathrm{MSD}}{50} \ldots . . . . . .(1) \
& \text {Now, } | \text { zero error }|=| 0-4(\mathrm{L} . \mathrm{C}) | \
& 0.04=|-4(\mathrm{L} . \mathrm{C})| \Rightarrow \mathrm{L} . \mathrm{C}=0.01 \mathrm{mm} \
& \mathrm{0 . 0 4}=|-\mathrm{4}(\mathrm{L} . \mathrm{C})| \Rightarrow \mathrm{L} . \mathrm{C} \ldots . . .(2) \
& =\mathrm{0 . 0 1} \mathrm{mm}
\end{aligned}Using(1)and(2)\begin{aligned}
& \frac{M S D}{50}=0.01 \
& M S D=0.5 \mathrm{mm} \
& M S D=\frac{1 \mathrm{cm}}{\mathrm{N}} \
& \mathrm{N}=\frac{1 \mathrm{cm}}{0.5 \mathrm{mm}}=\frac{100 \mathrm{mm}}{5 \mathrm{mm}} \
& \mathrm{N}=20
\end{aligned}$