XI. When the Vicar shall require a meeting of bishops, two from each province will be sufficient.
In summoning bishops to your presence, we wish you to show great forbearance: lest under a show of much diligence you seem to exult in your brethren's injuries. Wherefore if any greater case arise for which it is reasonable and necessary to convene a meeting of brethren, it may suffice, brother, that two bishops should attend from each province, whom the metropolitans shall think proper to be sent, on the understanding that those who answer the summons be not detained longer than fifteen days from the time fixed.
Source: Letters (New Advent)