A board seat is the highest-leverage governance right an investor can have. Board members vote on corporate actions: hiring/firing the CEO, approving budgets, approving financings, approving acquisitions. Lead investors at Series A typically take one board seat; Series B and later rounds usually expand the board further.
Board composition matters more than board count. A 5-person board with 3 independent directors is structurally different from a 5-person board with 3 investor directors. Founders typically lose board control between Series A and Series C; the goal is to maintain operational control through other governance mechanisms.