How many Mahajanapadas are mentioned in ancient Buddhist and Jain texts?

In the 6th century BCE, northern India saw the rise of large territorial states called Mahajanapadas, meaning "great realms". The Buddhist text Anguttara Nikaya and the Jain text Bhagavati Sutra both list sixteen Mahajanapadas, though the names in the two lists differ slightly.

The sixteen Mahajanapadas of the Buddhist list, with their capitals, were:

Anga (Champa), Magadha (Rajagriha, later Pataliputra), Kashi (Varanasi), Kosala (Shravasti), Vajji (Vaishali), Malla (Kushinara and Pava), Chedi (Shuktimati), Vatsa (Kaushambi), Kuru (Indraprastha), Panchala (Ahichchhatra and Kampilya), Matsya (Viratanagara), Shurasena (Mathura), Ashmaka (Potali, the only one south of the Vindhyas, on the Godavari), Avanti (Ujjayini and Mahishmati), Gandhara (Taxila) and Kamboja (Rajapura).

Most of them were monarchies, but some, such as Vajji and Malla, were ganas or sanghas, ruled by assemblies of clan leaders; these are often called early republics.

The rise of the Mahajanapadas was helped by the spread of iron tools, which made it easier to clear forests and expand farming in the Ganga valley, producing a surplus. Towns, trade and the use of punch-marked coins grew at this time. It was also the age of the Buddha and Mahavira.

The Mahajanapadas fought each other for supremacy. Four became powerful: Magadha, Kosala, Vatsa and Avanti. Eventually, Magadha, under rulers such as Bimbisara and Ajatashatru of the Haryanka dynasty, followed by the Shishunagas and the Nandas, became the most powerful. It later grew into the Mauryan Empire under Chandragupta Maurya.

Why not the other options?

Twelve, eighteen and twenty-four are incorrect. Twenty-four is the number of Jain Tirthankaras.