Although it is highly recommended to arrive early to Jumuah Prayer (Friday Prayer) it sometimes happens that a person may be running late. Often times people say that the khutba (sermon) is part of the prayer on Friday and tell others that if they miss the sermon they should pray 4 raka’at instead of 2.
According to the Hanafi school, namely in this case Abu Hanifa and Abu Yusuf, if the congregant arrives to prayer even as late as the final sitting then they pray 2 raka’ats with the intention of it counting as jumuah prayer. This is mentioned in many of the books of the madhhab such as Maraqi al-Falah, al-Lubab, and Hashiyah Ibn Abidin.
Muhammad ibn al-Hasan (from the top students of Abu Hanifa) was of the opinion that if one catches the ruku’ of the second raka’ah they caught the jumuah and pray 2. This is also the position of the Maliki school. In both cases missing the sermon does not affect the number of units prayed.
*I don’t know if there are other opinions in the other schools. Perhaps I will check and update the article later.