The Torah tells us (Exodus 12:15-16) to observe Passover for seven days and that the first day and seventh days will be a "sacred occasion." (Yom Tov) On the first day, the fifteenth day of the Hebrew month of Nisan. the seder is held.
When the Torah was written, the beginning of the new month was determined by observing the moon. This was done in Jerusalem. Word of the new moon did not always arrive in other cities outside of Israel in time to observe the holiday. For communities outside of Israel, the practice developed of observing an extra day of Yom Tov on major holidays. This led to holding two seders in the Diaspora. While Reform Jews observe the first day of Passover as a Yom Tov, most do not observe the second day as a Yom Tov as well. And while we should conduct or attend at least one seder during Passover, there is no maximum to the number of seders we can hold.