If I entered the prayers on Friday and the Imam had already started the prayer. Do I pray two Rak'ah or do I have to pray four Rak'ah?
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.
Missing the Khutba does not have any impact on the validity of the Friday prayer itself, although the Khutba is very important and attending it makes the person entitled to a great reward.
So, if you come while the Imam is still in the first Rak'ah, you join in the prayer and complete with the Imam the two Rak'ahs and pronounce the Salaam just after the Imam. If you miss the first Rak'ah with the Imam, you pray the second Rak'ah with the Imam and when Imam pronounces Salaam stand and make up for the Rak'ah you missed.
However, if you miss both Rak'ah then you pray four Rak'ah according to the regular Zuhr prayer.
Allah knows best.
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