A person who goes to attend Friday prayer can pray as many nafls as he wishes after the sun has declined from its zenith to when the Imam comes to give Khutbah. Any how he is expected to pray at least 2 rakats sunnat.
LISTENING TO KHUTBAH (SERMON)
Once the Khutbah starts, the whole congregation should listen to it in silence. If a person arrives while the Imam is giving Khutbah then this person should pray 2 rakats nafl before sitting down to listen to Khutbah.
Jabir (ra) said that the Messenger of Allah (saw) said while he was giving Khutbah:
"If anyone of you goes to attend the Friday Prayer while the Imam is delivering Khutbah he should pray 2 rakats and should not make them long."(Muslim)
There is another hadith Jabir (ra) says that once a man, came to Friday Prayer while the Messenger of Allah (saw) was delivering Khutbah, so Allah's Messenger (saw) asked him, "Did you pray ?" "No", he answered. Then the Prophet (saw) said to him, "Stand up and Pray>"(Bukhari, Muslim)
It is a continuous practice in some mosques that those who arrive while Imam is giving speech sit down and listen to the speech. When the Imam has finished the speech he gives time to the late arrivals to Pray 2 or 4 rakats sunnat. After that the Imam gives a short Khutbah in Arabic before praying the JUMAH Prayer.
Some people get very annoyed if they see a person offer 2 rakats sunnat, while the Imam is giving speech. They feel that the person is being disrespectful to the Imam. This is incorrect and unproved from the practice of the Prophet Muhammad (saw). It is also against those hadiths which we mentioned above and the one we are mentioning below.
Abl Qatadah (ra) says that the Messenger of Allah (saw) said, "Whenever one of you enters the mosque he should not sit down without offering 2 rakats."(Bukhari, Muslim)
These hadiths clarify the points which are erroneously practiced above.