Objection 1. It seems that there are not many angels in one order. For it was shown above (Question 50, Article 4), that all the angels are unequal. But equals belong to one order. Therefore there are not many angels in one order.
Objection 2. Further, it is superfluous for a thing to be done by many, which can be done sufficiently by one. But that which belongs to one angelic office can be done sufficiently by one angel; so much more sufficiently than the one sun does what belongs to the office of the sun, as the angel is more perfect than a heavenly body. If, therefore, the orders are distinguished by their offices, as stated above (Article 2), several angels in one order would be superfluous.
Objection 3. Further, it was said above (Objection 1) that all the angels are unequal. Therefore, if several angels (for instance, three or four), are of one order, the lowest one of the superior order will be more akin to the highest of the inferior order than with the highest of his own order; and thus he does not seem to be more of one order with the latter than with the former. Therefore there are not many angels of one order.
On the contrary, It is written: "The Seraphim cried to one another" (Isaiah 6:3). Therefore there are many angels in the one order of the Seraphim.
I answer that, Whoever knows anything perfectly, is able to distinguish its acts, powers, and nature, down to the minutest details, whereas he who knows a thing in an imperfect manner can only distinguish it in a general way, and only as regards a few points. Thus, one who knows natural things imperfectly, can distinguish their orders in a general way, placing the heavenly bodies in one order, inanimate inferior bodies in another, plants in another, and animals in another; whilst he who knows natural things perfectly, is able to distinguish different orders in the heavenly bodies themselves, and in each of the other orders.
Now our knowledge of the angels is imperfect, as Dionysius says (Coel. Hier. vi). Hence we can only distinguish the angelic offices and orders in a general way, so as to place many angels in one order. But if we knew the offices and distinctions of the angels perfectly, we should know perfectly that each angel has his own office and his own order among things, and much more so than any star, though this be hidden from us.
Reply to Objection 1. All the angels of one order are in some way equal in a common similitude, whereby they are placed in that order; but absolutely speaking they are not equal. Hence Dionysius says (Coel. Hier. x) that in one and the same order of angels there are those who are first, middle, and last.
Reply to Objection 2. That special distinction of orders and offices wherein each angel has his own office and order, is hidden from us.
Reply to Objection 3. As in a surface which is partly white and partly black, the two parts on the borders of white and black are more akin as regards their position than any other two white parts, but are less akin in quality; so two angels who are on the boundary of two orders are more akin in propinquity of nature than one of them is akin to the others of its own order, but less akin in their fitness for similar offices, which fitness, indeed, extends to a definite limit.
Source: Summa Theologica (New Advent)