Objection 1. It would seem that each man is not guarded by an angel. For an angel is stronger than a man. But one man suffices to guard many men. Therefore much more can one angel guard many men.
Objection 2. Further, the lower things are brought to God through the medium of the higher, as Dionysius says (Coel. Hier. iv, xiii). But as all the angels are unequal (50, 4), there is only one angel between whom and men there is no medium. Therefore there is only one angel who immediately keeps men.
Objection 3. Further, the greater angels are deputed to the greater offices. But it is not a greater office to keep one man more than another; since all men are naturally equal. Since therefore of all the angels one is greater than another, as Dionysius says (Coel. Hier. x), it seems that different men are not guarded by different angels.
On the contrary, On the text, "Their angels in heaven," etc. (Matthew 8:10), Jerome says: "Great is the dignity of souls, for each one to have an angel deputed to guard it from its birth."
I answer that, Each man has an angel guardian appointed to him. This rests upon the fact that the guardianship of angels belongs to the execution of Divine providence concerning men. But God's providence acts differently as regards men and as regards other corruptible creatures, for they are related differently to incorruptibility. For men are not only incorruptible in the common species, but also in the proper forms of each individual, which are the rational souls, which cannot be said of other incorruptible things. Now it is manifest that the providence of God is chiefly exercised towards what remains for ever; whereas as regards things which pass away, the providence of God acts so as to order their existence to the things which are perpetual. Thus the providence of God is related to each man as it is to every genus or species of things corruptible. But, according to Gregory (Hom. xxxiv in Evang.), the different orders are deputed to the different "genera" of things, for instance, the "Powers" to coerce the demons, the "Virtues" to work miracles in things corporeal; while it is probable that the different species are presided over by different angels of the same order. Hence it is also reasonable to suppose that different angels are appointed to the guardianship of different men.
Reply to Objection 1. A guardian may be assigned to a man for two reasons: first, inasmuch as a man is an individual, and thus to one man one guardian is due; and sometimes several are appointed to guard one.
Secondly, inasmuch as a man is part of a community, and thus one man is appointed as guardian of a whole community; to whom it belongs to provide what concerns one man in his relation to the whole community, such as external works, which are sources of strength or weakness to others. But angel guardians are given to men also as regards invisible and occult things, concerning the salvation of each one in his own regard. Hence individual angels are appointed to guard individual men.
Reply to Objection 2. As above stated (112, 3, ad 4), all the angels of the first hierarchy are, as to some things, enlightened by God directly; but as to other things, only the superior are directly enlightened by God, and these reveal them to the inferior. And the same also applies to the inferior orders: for a lower angel is enlightened in some respects by one of the highest, and in other respects by the one immediately above him. Thus it is possible that some one angel enlightens a man immediately, and yet has other angels beneath him whom he enlightens.
Reply to Objection 3. Although men are equal in nature, still inequality exists among them, according as Divine Providence orders some to the greater, and others to the lesser things, according to Sirach 33:11-12: "With much knowledge the Lord hath divided them, and diversified their ways: some of them hath He blessed and exalted, and some of them hath He cursed and brought low." Thus it is a greater office to guard one man than another.
Source: Summa Theologica (New Advent)