There were 25 cardinals in the College of Cardinals in July 1216, including 23 curial cardinals and two "external cardinals", who did not reside in the papal curia It is known that 17 of them participated in the election:
The cardinals assembled in Perugia two days after the death of Innocent III. They deliberated in the enclousure, though it is not certain whether volunatarily or under pressure of the local authorities. They decided to elect the new Pope by compromissum, it means, not by the whole Sacred College, but by the committee of few of them, empowered by the rest to appoint the new Pontiff. This time the committee included only two cardinal-bishops: Ugolino of Ostia and Guido of Palestrina. On that same day, they elected Cardinal Cencio, called Camerario, aged 68, who accepted his election and took the name of Honorius III.
^The older sources refer him as member of Savelli family but modern scholars reject it 
^The reconstruction of the Sacred College in July 1216 is based on W. Maleczek, Papst und Kardinalskolleg von 1191 bis 1216, Wien 1984. Maleczek has rectified the earlier reconstructions made by Alphonso Ciacconio in his Vitae et res gestae Pontificum Romanorum et S. R. E. Cardinalium (1677) and Conrad Eubel in his Hierarchia Catholica Medii Aevi (1913), which are the main sources for the lists in both electronic accounts listed below.
^Pierre Duacensis and Peter of Benevento were actually different persons; Duacensis was never promoted to the cardinalate. See an article by John Wei, TWO LETTERS OF HONORIUS III TO CATALONIA, p. 84-85 note 6