Nicaea 325 AD and Arius?

The number that attended Nicaea is disputed, however all but two voted against Arius.

Constantine had invited all 1800 bishops of the Christian church (about 1000 in the east and 800 in the west), but only 250 to 320 bishops actually participated. Eusebius of Caesarea counted 250 (Eusebius of Caesaria, Life of Constantine, Clarendon Ancient History series), Athanasius of Alexandria counted 318, (Athanasius, Ad Afros Epistola Synodica), and Eustathius of Antioch counted 270 (Theodoret H.E. 1.7). All three were present at the council. Later, Socrates Scholasticus recorded more than 300, (H.E. 1.8) and Evagrius, (H.E. 3.31) Hilarius, (Contra Constantium) JeromChronicon e (Chronicon) and Rufinus recorded 318.

Philip Schaff concludes,

The CONCILIUM NICAENUM I., A.D. 325; held at Nicaea in Bithynia, a lively commercial town near the imperial residence of Nicomedia, and easily accessible by land and sea. It consisted of three hundred and eighteen bishops, [This is the usual estimate, resting on the authority of Athanasius, Basil (Ep. 114; Opera, t. iii. p 207, ed. Bened.), Socrates, Sozomen, and Theodoret; whence the council is sometimes called the Assembly of the Three Hundred and Eighteen. Other data reduce the number to three hundred, or to two hundred and seventy, or two hundred and fifty, or two hundred and eighteen; while later tradition swells it to two thousand or more.] besides a large number of priests, deacons, and acolytes, mostly from the East, and was called by Constantine the Great, for the settlement of the Arian controversy. Having become, by decisive victories in 323, master of the whole Roman empire, he desired to complete the restoration of unity and peace with the help of the dignitaries of the church. The result of this council was the establishment (by anticipation) of the doctrine of the true divinity of Christ, the identity of essence between the Son and the Father. The fundamental importance of this dogma, the number, learning, piety and wisdom of the bishops, many of whom still bore the marks of the Diocletian persecution, the personal presence of the first Christian emperor, of Eusebius, "the father of church history," and of Athanasius, "the father of orthodoxy" (though at that time only archdeacon), as well as the remarkable character of this epoch, combined in giving to this first general synod a peculiar weight and authority. It is styled emphatically "the great and holy council," holds the highest place among all the councils, especially with the Greeks,[For some time the Egyptian and Syrian churches commemorated the council of Nicaea by an annual festival.] and still lives in the Nicene Creed, which is second in authority only to the ever venerable Apostles' Creed. This symbol was, however, not finally settled and completed in its present form (excepting the still later Latin insertion of filioque), until the second general council. Besides this the fathers assembled at Nicaea issued a number of canons, usually reckoned twenty on various questions of discipline; the most important being those on the rights of metropolitans, the time of Easter, and the validity of heretical baptism.

(Philip Schaff and David Schley Schaff, History of the Christian Church, Oak Harbor, WA: Logos Research Systems, Inc., 1997).

Additional References:

Bruce L. Shelley, Church History in Plain Language, Updated 2nd ed., 115 (Dallas, Tex.: Word Pub., 1995).

Paul Lagass and Columbia University, The Columbia Encyclopedia, 6th ed. (New York; Detroit: Columbia University Press; Sold and distributed by Gale Group, 2000).