How Many Verses Should Be Recited after Al-Fatihah?
Question

I would like to ask you about the second short Surah in the prayer. What is the minimum number of verses that it is permissible to recite?
Praise be to Allah.Is it obligatory to recite a Surah after Al-Fatihah?
Reciting a Surah after Al-Fatihah in the first two Rak`ahs of the prayer is recommended, not obligatory, according to the majority of scholars.
Abu Hurayrah (may Allah be pleased with him) said: 
“In every prayer there is recitation, so what the Prophet (blessings and peace of Allah be upon him) made us hear we make you hear, and what he recited quietly we recite quietly. Whoever recites the Essence of the Book (i.e., Al-Fatihah ), that will suffice him, but whoever does more than that, that is better.” (Narrated by Al-Bukhaari, 738 and Muslim, 396)
An-Nawawi (may Allah have mercy on him) said: 
“The words “Whoever recites the Essence of the Book (i.e., Al-Fatihah), that will suffice him, but whoever does more than that, that is better” indicate that it is obligatory to recite Al-Fatihah and that the prayer is not valid without it.
It also indicates that reciting a Surah after it is recommended. There is consensus on that with regard to Fajr, Jumu`ah and the first two Rak`ahs of every prayer, and it is Sunnah according to all scholars. Al-Qadi `Iyad (may Allah have mercy on him) narrated from some of the companions of Malik that it is obligatory to recite a Surah after Al-Fatihah, but this is an odd view and is to be rejected.” (Sharh Muslim (4/105)
How many verses should be recited after Al-Fatihah?
The minimum that may be recited is one verse. Imam Ahmad (may Allah have mercy on him) recommended that it should be a lengthy verse, such as the verse on debt (Al-Baqarah 2:282) or Ayat Al-Kursi (Al-Baqarah 2:255).
It says in Sharh Al-Muntaha (1/191): Al-Qadi (Abu Ya`la) and others said: “It is sufficient to recite one verse, except that Ahmad recommended that it should be a lengthy verse, such as the verse on debt (Al-Baqarah 2:282) or Ayat Al-Kursi (Al-Baqarah 2:255).”
If the worshipper decides to recite only one verse, it should be a verse that conveys a complete meaning or a ruling when recited on its own.
Al-Buhuti (may Allah have mercy on him) said in Kashshaf Al-Qina` (1/342): What appears to be the case is that it is not sufficient to recite a verse that does not convey a meaning or ruling when recited on its own, such as (interpretation of the meaning)
“Then he considered [again]” [Al-Muddaththir 74:21]
Or
“Dark green [in colour]” [Ar-Rahman 55:64].” 
For more details, please see the following answers: 20043 , 69915 , 171060 , and 7198 .
And Allah knows best.