New Blood
From Halopedia, the Halo wiki
New Blood is an upgrade in Halo Wars, aquired from the barracks, which adds one extra marine to every marine unit, increasing the total to 5 marines. It costs 200 supplies, requires a tech level of 1 and takes roughly 20 to completed. This upgrade increases both the damage that the unit can deal as there are more guns trained on a single target, as well as increasing the overall health, since there are more people to be killed.
This upgrade is permanant and will effect any marine units on the battlefield as well as be applied to all marine units that are yet to be trained. It will also remain in effect, as long as the tech level is high enough to train the original units.