Terawih is usually prayed in pairs of two and can be prayed in any amount of pairs, although eight or 20 pairs is the general practice among Muslims.
Prophet Muhammad regarded the terawih prayer as a means of forgiveness of sins. He said, “The one who observes the terawih prayer at night during Ramadan with complete faith and devotion only for the sake of the recompense of the hereafter will have all of his previous sins forgiven by God.”
Sunni Muslims believe terawih is a sunnah (a practice of the Prophet) and may be performed at home if one is unable to attend a mosque. According to this tradition, the Prophet Muhammad initially prayed the terawih in congregation during Ramadan but later discontinued this practice out of fear that Muslims would start to believe the prayers were mandatory.
Shiite Muslims do not perform terawih because its practice was institutionalized by Umar, the second caliph -- whose rule Shiites view as illegitimate.