Technically no.
Missions are designed to be scaled for almost any party size (I've both run and played in groups as small as 3 players and as large at 9 at conventions) and any party composition. It's on the GM to maintain challenge without overwhelming the current party by reducing (or increasing) the difficulty of challenges.
If the players feel they need extra manpower, several Missions NPCs can be hired on a Mission by Mission basis to provide some extra muscle or skills (see the current FAQ).
Bull