The ship's level caps how strong an army it carries; deckhands are never capped.
1«Might X/Y» in the hire window: X is your army now, Y the most your ship takes. Might is the army's boarding strength counted in deckhands.
2«Picked» is a second limit: tier 4 and up may fill only part of the hammocks (from 15 % at ⚓1 to 45 %).
3Y grows with the ship's level, its hammocks and its slots: a new hull or a refit raises it. When the army is as strong as the ship allows, the game says so and the hire stops; deckhands and titans are never capped.