Is it must for a Muslim to follow any one of four "Madhab Imam"? How many "Madhab" Imams are there in total?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

As for a general man who does not have any knowledge about the evidences of Shariah and does not know how to give preference to a given evidence over another. Such a person should follow the Mufti who issues a Fatwa to him. The Madhab of such a person is the same as the Madhab of his Mufti. But, he is not obliged to follow a certain Mufti. Such a person should refer to any available Mufti who has sound knowledge whenever he needs to ask about religious matters. If he gets informed about another good knowledgeable person then he should also refer to that Mufti in order to seek the knowledge from such a person too.
2. Only the four known Madahib (Schools of thought) are popular among the Muslim Ummah, i.e. Ahl-ul-Sunnah. Those Madahib are the Madhab al-Hanafi, al-Shafi, al-Maliki and al-Hambali.
For more details please read the Fatawa:
 81331, 81976 and 82570.
Allah knows best.