Take your tank as an example, up to four, because crayfish are generally above 5cm, and crayfish are very territorial. I suggest you keep two, and it's better to eat less. More sunken wood, stones and pipes are arranged to provide them with larger activity area and residence.
In addition, crayfish are suitable for raising in alkaline water to facilitate their molting (acidic water is not conducive to shrimp molting, and the chemical principle needs no explanation). Putting stones and coral bones can increase acidity. If Shui Mu is used as decoration, coral bones can be added to the filter material, which can not only enhance the filtration, but also reduce the acidity.
Also, crayfish should not be mixed with small fish and shrimp, because crayfish are omnivorous. Alas, they eat meat, and no amount of small fish and shrimp is enough for him to eat. Consider some medium-sized fish, such as Manlong, Han shark (Genghis Khan/white shark) and blue shark. If you have two or three crayfish in your tank, it's good to have two Han sharks.
Dream of lobster (individual), the blue devil in the sky.