Unfortunately we do not have the text of the letter from Constantine which convoked the Council of Nicea. Eusebius tells us that the emperor sent each bishop a very respectful letter requesting that he come to Nicea. Since it was an imperial request it was difficult to disregard. Nevertheless not all sees were represented by their bishops. Rome, for instance, sent delegates to represent the Pope.
Traditions differ as to the number of bishops who actually attended.
The one most commonly accepted is 318. Apparently all parts of the empire were represented, including Persia. We are not sure whether the Council actually opened on May 20 or June 19th, 325. According to Eusebius the emperor not only convoked the council. He presided, appearing regal in golden attire covered with jewels.