In Dark Crusade, you only need 5 population's worth of Orks within a 15 unit radius to invoke a basic Mob Bonus, other prerequisites notwithstanding. The Bonus is applied to whole squads, and not to individuals. So, the larger your squad, the more perks it enjoys. And remember, Bosses and Nobs count for 2 Ork pop each.

At its simplest, the Bonus at "level one" is a simple morale booster. In other words, the rate at which Ork squads regain their morale begins to grow, and that rate improves the more Boyz there are in a squad. For every member in an Ork squad, there's a 7% morale recharge boost. For a fully reinforced, 15 man Slugga Squad, that can snowball to a pretty hefty number: 2.75 times the normal morale healing rate.

If you have at least 30 pop's worth of Orks within a 20 unit radius (e.g. two fully reinforced Slugga squads), you hit Mob Bonus "level two". The Mob become morale immune, just like Chaos Bezerkers. Nothing will break their morale or cause them to panic while they can maintain these numbers in their mob.

Of course, that's never as easy as it sounds with the Orks - losses and high attrition rates can quickly unravel a Mob. If you want to take advantage of the Mob Bonus, you'll want to send your Orks in waves to your opponent, rather than just dribble them in to fight.

At the same time, the Mob Bonus has a "level three". It triggers very early, when you only have 5 population's worth of Orks within a 25 unit radius.

They gain a 2% attack bonus for every extra ork in the mob, which stacks nicely with the other Mob perks. You can imagine what this sort of incremental bonus can do to a wall of Big Shoota Boyz or a mass of free Sluggas... ;)

The practical upshot of all this is - the bigger the Mob, the more fearless and meaner it becomes!