Space Marines Population

Population is a fixed budget that structures how large an army you can field and what kind of army you can field.

For most races population works the same way. Differences in population cost create asymmetry in the matchup.

There are two kinds of population: Squad Cap and Support Cap. Squad Cap is usually used for Infantry. Support Cap is usually used for vehicles.

Every race has a starting amount of population. To increase this further the player either: Gaining population as you tier encourages maxing out the army population at each tier before progressing to the next tier. Gaining population for the 2nd HQ encourages the same for an economic boom strategy. Researches that provide population (and nothing else) are effectively a tax; a decision made to indicate that you either want more population quickly or have consumed other sources already (i.e. you're in the late game).
 * Upon researching a series of progressively more expensive researches that increase the population for either Squad or Support Cap
 * Upon researching Tier 2 and Tier 3
 * Building the 2nd HQ, Orbital Relay or Vehicle Bay

Space Marine Infantry
For context, the initial Squad Population of the Space Marines is 9. It is typical for Heavy Infantry ranged units to cost at least 3 pop, with the exception being Battle sisters. Non-Eldar Light Infantry, especially those that are melee, tend to cost 2 pop. Example: Kroot. This allows them, under circumstances, to achieve a squad numbers advantage.

When you don't want to incur the costs to increase population further, which can be expensive, consider constructing: Scouts and Terminators are synergistic not only because they have different armour types but also because the low population cost of Scouts offsets the high population cost of Terminators.
 * Apothecaries
 * Librarian
 * Chaplain
 * Skull Probe

In the early game Space Marine players tend to buy 3x Scouts, which cost 3 pop in total. As mentioned earlier, the initial squad pop is 9, so you are left with 6 pop remaining. This gives you room to buy 2 squads of infantry that each cost 3 pop; such as either Tactical Space Marine, Assault Marine or Grey Knight. This is in addition to the Force Commander, which costs no population and is a highly recommended buy.

In highly aggressive games it is common for Scouts to be lost in battle. This gives you an immediate army composition choice for the early-mid game:
 * Replace the Scouts in order to have more squads and to have a greater ability to contest the map
 * Instead buy a 3 population unit in order to have more brute force.

Space Marine Vehicles
Space Marine's initial support population is 2. This is increased by a further +2 upon construction of the Vehicle Bay. Gradually increasing Support Population enables decisions between varied combinations of vehicles.

Total Population
To put the above costs into perspective, it is worth knowing how much population the SM have throughout the game.

In addition to the above, each Support Cap increase or Squad Cap Increase research increases their respective cap by 4.