Hi
Thank you for trying out the game.
Currently there are 2 ways to increase crew slots.

1. By leveling up
   Formula: 6 + floor((level - 1) / 3) * 3 (capped at 18)
2. Unlocking zones
   Velvet Square +3 slots
   Neon Bazaar +4 slots
   Ember Ward +5 slots
   Glass District +6 slots
   Throne Heights +7 slots
   Void Reach +8 slots
   Apex Enclave +10 slots

Good news is, the daily free crews stack till premium is active. Hopefully you will be able to level up and unlock some slots unless you are tempted to try out new zones 😀