If you use more than one or many cannabis seeds in a pot, the cannabis plants will begin to compete against each other for nutrients, so the smallest cannabis plants (which sometimes may be the best phenotypes.) will get wiped out by the bigger cannabis plants.
If you are growing from regular cannabis seeds they could also cross-pollinate resulting in buds with cannabis seeds in them. It sounds simple enough but if the buds have seeds inside them, the potency of the strain can be reduced up to 30%. Each cannabis seed is a plant and they need their own space to grow and thrive, as well as to produce bigger and better buds. Remember that more than one cannabis seed per pot is too many.