The practice from the Messenger of Allaah - sallallaahu alahyi wa sallam - was that he would leave his house on the day of Jumu'ah and whenever he got to the masjid, he would mount the mimbar, the muadhin would make the adhaan and after that he - sallallaahu alahyi wa sallam - would begin his speech.
The only naafilah is tahiyyatul-masjid (prayer of greeting the mosque). This, for the rest of the comers to Jumu'ah not the Imaam. In the case of the latter, he goes straight and mounts the mimbar.
Therefore, for the rest of the people, there is no recommended naafilah except the usual tahiyyatul-masjid which is ordinarily an obligation (though there is a difference of opinions as regard its being obligatory), Jumu'ah or not. Even at that, the Imaam is exempted.
However, the period before the arrival of the Imaam, since the rising above the spear of the sun, is open for general naafilah, without a specific number, just as it is open for tasbeehaat, tahleelaat, tilaawah qur'aan, salaatu alaa nabiyy, etc.
Reference: See partly: Al-Qawl Al-Mubeen fee Akhtaaeel Musalleen, pp.359-69
Next Edition: Status of Tahiyyatul-masjid.
Also watch out for the recommended naafilah after Jumu'ah.
In the past edition: