The "normal" requirement to qualify for group perks is a minimum of 8 cabins, sometimes it requires 10.
For every 8 people booked you should be able to get the equivalent of value of 1 free berth. So.. 8 cabins booked, you'd get value of 2 berths.
PLUS there are normally additional perks available to choose from, such as shipboard credits, cocktail parties, photo credits, etc.
My advice is to find a travel agent experienced in group bookings and work with them to set up a package of group pricing and perks that satisfies you.