The unique Rosh Hashanah Mussaf Amidah contains nine blessings. It keeps the three opening blessings and three closing blessings found in every Amidah, but expands the usual single middle blessing into three special sections.
Those central sections are Malchuyot (Kingship), Zichronot (Remembrance), and Shofarot. Each develops a theme through biblical passages and connects the holiday with divine judgment, memory, sovereignty, and the shofar.
On most days when Mussaf is recited, the Amidah has seven blessings: three at the beginning, one central blessing about the day, and three at the end. Rosh Hashanah is therefore unusual not because it has a different opening or conclusion, but because its central liturgy is divided into three.
The number can be confused with the eighteen or nineteen blessings of the weekday Amidah. Those counts describe another prayer structure; the Rosh Hashanah Mussaf Amidah is a special festival form. The shofar is traditionally sounded during the cantor’s repetition after the sections connected with these three themes, except when the holiday coincides with Shabbat.