Are two rak'ahs of Sunnah prayer performed before Friday prayer and four after it, and how does one thereby achieve the twelve rak'ahs of regular Sunnah prayers that the Prophet (peace and blessings of Allah be upon him) recommended?

The two rak'ahs before the Friday prayer are not included in the regular supererogatory prayers (Sunan al-Rawatib) mentioned in the Hadith: "Whoever prays twelve rak'ahs in a day and night, a house will be built for him in Paradise thereby," because there is no specific number for a regular pre-Jumu'ah supererogatory prayer. However, it is desirable for one who enters the mosque on Friday to pray, without specifying a number. As for the supererogatory prayer after Jumu'ah, it is among the regular supererogatory prayers.