What is the minimum number of people required to form a jamaath? (Any farz namaz)
Narrated Ibn Abbas:
(One night) I stayed overnight in the house of my aunt Maimuna, and said to myself, "I will watch the prayer of Allah's Apostle " My aunt placed a cushion for Allah's Apostle and he slept on it in its length-wise direction and (woke-up) rubbing the traces of sleep off his face and then he recited the last ten Verses of Surat-al-Imran till he finished it. Then he went to a hanging water skin and took it, performed the ablution and then stood up to offer the prayer. I got up and did the same as he had done, and stood beside him. He put his hand on my head and held me by the ear and twisted it. He offered two Rakat, then two Rakat, then two Rakat, then two Rakat, then two Rakat, then two Rakat, and finally the Witr (i.e. one Rak'a) prayer.
Volume 6, Book 60, Number 94:Sahih Bukhari
Thus from this we can derive that 2 people are enough to offer in a jam'ah
Also please see this hadith as well:
Volume 1, Book 12, Number 782: Narrated Abu Qilaba:
Once Malik bin Huwairith said to his friends, "Shall I show you how Allah's Apostle used to offer his prayers?" And it was not the time for any of the compulsory congregational prayers. So he stood up (for the prayer) bowed and said the Takbir, then he raised his head and remained standing for a while and then prostrated and raised his head for a while (sat up for a while). He prayed like our Sheikh 'Amr Ibn Salama. (Aiyub said, "The latter used to do a thing which I did not see the people doing i.e. he used to sit between the third and the fourth Rak'a). IMalik bin Huwairith said, "We came to the Prophet (after embracing Islam) and stayed with him. He said to us, 'When you go back to your families, pray such and such a prayer at such and such a time, pray such and such a prayer at such and such a time, and when there is the time for the prayer then only of you should pronounce the Adhan for the prayer and the oldest of you should lead the prayer."