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As you explore the enormous universe of Starfield, you’ll come across a number of recruitable NPCs which you can add to the crew of your ship. Your crew accompanies you during voyages and can also be assigned work on the ship. The first ship you get, the Frontier, has a limited crew capacity of 2, but there are ways to increase this. Here is all you need to know about how to increase your crew size in Starfield.
How to Increase Crew Size in Starfield
Crew Capacity (CC) is the maximum amount of crew members you can assign to your ship and the Crew Station (CS) is the maximum amount of crew members that can work on your ship. CS numbers can be increased through the cockpit and hab modules. Some examples of cockpits and habs that increase your crew capacity in Starfield are Kon-Tiki B-500 Bridge and Stroud Battle Station 2x2, both found in Stroud-Ecklund shop in Neon.
Crew capacity can be increased through upgrading your ship systems like reactors, shields, weapons and engines. SF, Pinch, or Fusor reactor variants give you a +3 crew capacity while every weapon grants 0.5 crew capacity. Shields also add 0.5 crew capacity to your ship. You can see these numbers and adjust them by going into ship building. Players can experiment with different combinations to optimize their ship.

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To Increase crew size in Starfield, you need to build more crew stations in your outpost. You can find these in the miscellaneous section of Build Mode. Each crew station requires 5x Aluminum, 2x Iron and 3x Nickel to make.
Note that if you have 2 CC and 5 CS you'll only be able to assign 2 members because you're bottlenecked by CC. Inversely if you have 5 CC and 3 CS you'll only be able to assign 3 members. You need an equal amount of CC and CS, or at least enough of both to assign your desired members. You can also increase the crew size by upgrading the Ship Command skill. The Ship Command skill is in the final tier of the skills in the 'Social' category. Each rank of this skill increases the maximum number of allowed crew members by +4/5/6/8. You can level up the skill by destroying or boarding vessels while having multiple crew members on your ship.
To increase your Ship crew capacity in Starfield, you need to upgrade different areas of your ship with modules that increase your maximum crew capacity and you should increase your 'Ship Command' skill to its highest rank. However, you first need to know how to figure out your current crew capacity.