Short answer: In a normal human, there is one aorta — a single large artery that carries blood from the left ventricle to the body.
Step-by-step explanation
1) Origin and overall path: The aorta arises from the left ventricle of the heart and proceeds as one continuous vessel. It is usually described in sections: the ascending aorta, the aortic arch, and the descending aorta (which includes the thoracic and abdominal aorta).
2) Major subdivisions and branches: Although it is one vessel, the aorta gives off many branches along its course (for example, the coronary arteries from the ascending aorta; the brachiocephalic, left common carotid and left subclavian arteries from the arch; and paired visceral and lumbar branches from the abdominal aorta). Near the lower abdomen the aorta divides (bifurcates) into the two common iliac arteries to supply the legs — this bifurcation is sometimes why people think of 'two' arteries downstream, but the aorta itself up to that point is a single vessel.
3) Why someone might ask "one or two": A few congenital variations can make it appear there are two aortic channels. The most notable is a double aortic arch, a developmental anomaly where two aortic arches form a vascular ring around the trachea and esophagus. Another variant is a right aortic arch (the aorta arches to the right instead of the usual left). These are uncommon and are considered congenital anomalies, not the normal anatomy.
4) Clinical relevance: Most people with a normal single aorta are asymptomatic. Double aortic arch or other significant anomalies can cause breathing or swallowing problems and are usually detected with imaging (echocardiography, CT, MRI) and treated if needed.
Summary: Normally there is one aorta. It branches many times and eventually splits into two common iliac arteries in the pelvis. Rare congenital variants can create two arch-like structures, but these are not the typical anatomy.