Answer
44 subjects
Work Step by Step
Given $E=2, \sigma=8$, at the 90% confidence, the critical z-value is $z_{\alpha/2}=1.645$,
and we have the equation $E=1.645\times\frac{8}{\sqrt n}=2$ which gives
the required sample size as $n=(\frac{1.645\times8}{2})^2\approx44$ (round up to the next integer here)