Answer
(a) Series.
(b) $N$ = 5000 Cells.
Work Step by Step
(a) In series, the total voltage of the cells is larger than the potential of the individual cell. Hence, to produce total potential more than 0.10 V, the cells must be connected in $\boxed{\text{series}}$.
(b) The number of cells $N$ equals the total potential divided by the potential of the individual cell
$$N = \dfrac{V_{total}}{V_{cell}} = \dfrac{500 \,\text{V}}{0.10 \,\text{V}} = \boxed{5000 \,\text{Cells}}$$
