How many people must pray together for the prayer to be considered a congregational prayer?
the Permanent Committee of Saudi Arabia for Fatwâ and Research
The congregational prayer requires at least two people. A child who is seven years old can be considered in congregation with one more person. However, it is much better for there to be a greater number of worshippers than this.
It is obligatory for men to go to the mosque to pray in the congregation there and not neglect
Source: Islam Today
-- Al Arabiya Digital