Rosh Hashanah is observed for two days in most Jewish communities.
The two-day observance covers 1 and 2 Tishrei. It developed partly because ancient Jewish authorities relied on witnesses reporting the sighting of the new moon, making it difficult for communities far from Jerusalem to know the exact date in time.
Although the Torah describes the festival as a one-day celebration, rabbinic practice established a second day. Rosh Hashanah is unusual because this additional day has traditionally been observed even in Israel, where many other biblical festivals are celebrated for one day.
The wording “most communities” matters. Many Reform congregations, especially in North America, observe only the first day, and Karaite Jews also keep one day. Thus, Two is the standard broad answer, not an absolute rule for every Jewish community.