Trachea divides into two right and left primary bronchi at the level of

Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of
5^th thoracic vertebra into a right and left primary bronchi.

Cervical region has 7 vertebrae. 7+5=12 which makes 5^th thoracic vertebra, the 12th one in the vertebral column.