How many muezzins there should be, one or two

Sayyiduna Ibn Umar (may Allah be pleased with them both) said: The Prophet (peace and blessings be upon him) had two mu'adhdhins who called the adhan. Ibn Numayr added in his narration: Sayyiduna Ibn Umm Maktum and Sayyiduna Bilal (may Allah be pleased with them both).

Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) had only one mu'adhdhin, who called the adhan when he (peace and blessings be upon him) sat on the pulpit and said the iqamah when he (peace and blessings be upon him) came down from the pulpit. It was the same in the time of Sayyiduna Abu Bakr and Sayyiduna Umar (may Allah be pleased with them both). When the time of Sayyiduna Uthman (may Allah be pleased with him) came, the people increased and spread out, so he added the third call at the time of zawal, or at al-Zawra'.