1) A person is obligated to make at least one hundred blessings every day. This was a decree from King David, when there was a plague in his days and exactly one hundred men were dying each day, he decreed that Klal Yisrael should make one hundred blessings a day, and the plague stopped. (Shulchan Aruch and Mishne Berurah 46)
2) On a regular weekday, a person anyway makes one hundred blessings. However, on Shabbos and festivals, where the prayer called the Amidah is reduced from nineteen blessings to seven, one should add blessings by eating fruit etc. If necessary, one can use the blessings said by the one called up to read in the Torah. (Mishne Berurah ibid)
