Bismillaah:

The minimum number that constitutes a congregation is two, the Imam and a person with him.

Abu Musa Al Ashaari رضي الله عنه said:

The Prophet صلى الله عليه وسلم said:

'' Two and more constitute a congregation.''

Takhreej:

{ Imam Ibn Majah in his Sunnah #972 and declared as Daeef by Imam al Busaayri رحمه الله and others due to two weak narrators Ar Rabi and his son Badr }

The more the congregation increases the more the prayer is beloved to Allaah

Ubayy ibn Ka’b رضي الله عنه said:

The Messenger of Allaah صلى الله عليه وسلم said:

“A man’s prayer with one other man is better than his praying alone, and a man’s prayer with two other men is better than his praying with one other. The more there are the more beloved that is to Allaah.’’

Takhreej:

{ Imam Al Hakim in his Mustadrak 3/269 and Ahmad in his Musnad and declared as saheeh by Imam Ibn Khuzaimah, Ibn Hibban, Al Albaani, Abu Muaadh رحمهم الله }

Another report:

The Prophet صلى الله عليه وسلم said:

“The prayer of two men, one of them leading the other, is better before Allaah than the prayer of eight praying separately, and the prayer of four men, one of them leading the others, is better before Allaah than the prayer of one hundred praying separately.”

Takhreej:

{ Imam Al Bazaar in his Musnad and declared as Hasan by Shaykh Al Albaani رحمهم الله }

Imam Ibn Hubairah رحمه الله commented:

" The Scholars have unanimously agreed that the least number that makes a congregation for an obligatory prayer, other than the prayer of Jumu'ah is two: an Imam and a follower standing on his right side.''

{ Al Insaaf 1/55 }

Imam Ibn Qudamah رحمه الله commented:

'' The congregation is considered to be such with two people and we know of no difference concerning this. If a person were to lead his slave or his wife, they would have attained the excellence of the congregation.''

{ Al Mughni }

*** And Allaah عزوجل knows best ***