Tefillah u’Birkas Kohanim 1:8

Thus we find that there are three daily prayer services: maariv, shacharis and mincha. There are four services on Shabbos, yom tov and rosh chodesh - the three that are recited each day, plus musaf. On Yom Kippur, there are five: these four, plus ne’ilah.

Tefillah u’Birkas Kohanim 1:9

The number of daily prayer services may not be decreased but they may be increased. If one wants to pray all day long, such is his prerogative. Any prayer that a person adds is considered a freewill offering. To do this, one must add, in each of the middle brachos of Shemoneh Esrei, a new idea that is in line with the theme that bracha. However, if he only adds a new idea in one bracha, this is sufficient to demonstrate that this is a voluntary prayer and not one of the obligatory prayers. One may never alter the text of the first three or the last three brachos of Shemoneh Esrei.