Objection 1. It would seem that one habit is made up of many habits. For whatever is engendered, not at once, but little by little, seems to be made up of several parts. But a habit is engendered, not at once, but little by little out of several acts, as stated above (Question 51, Article 3). Therefore one habit is made up of several.
Objection 2. Further, a whole is made up of its parts. Now many parts are assigned to one habit: thus Tully assigns many parts of fortitude, temperance, and other virtues. Therefore one habit is made up of many.
Objection 3. Further, one conclusion suffices both for an act and for a habit of scientific knowledge. But many conclusions belong to but one science, to geometry, for instance, or to arithmetic. Therefore one habit is made up of many.
On the contrary, A habit, since it is a quality, is a simple form. But nothing simple is made up of many. Therefore one habit is not made up of many.
I answer that, A habit directed to operation, such as we are chiefly concerned with at present, is a perfection of a power. Now every perfection should be in proportion with that which it perfects. Hence, just as a power, while it is one, extends to many things, in so far as they have something in common, i.e. some general objective aspect, so also a habit extends to many things, in so far as they are related to one, for instance, to some specific objective aspect, or to one nature, or to one principle, as was clearly stated above (2,3).
If then we consider a habit as to the extent of its object, we shall find a certain multiplicity therein. But since this multiplicity is directed to one thing, on which the habit is chiefly intent, hence it is that a habit is a simple quality, not composed to several habits, even though it extend to many things. For a habit does not extend to many things save in relation to one, whence it derives its unity.
Reply to Objection 1. That a habit is engendered little by little, is due, not to one part being engendered after another, but to the fact that the subject does not acquire all at once a firm and difficultly changeable disposition; and also to the fact that it begins by being imperfectly in the subject, and is gradually perfected. The same applies to other qualities.
Reply to Objection 2. The parts which are assigned to each cardinal virtue, are not integral parts that combine to form a whole; but subjective or potential parts, as we shall explain further on (57, 6, ad 4; II-II, 48).
Reply to Objection 3. In any science, he who acquires, by demonstration, scientific knowledge of one conclusion, has the habit indeed, yet imperfectly. And when he obtains, by demonstration, the scientific knowledge of another conclusion, no additional habit is engendered in him: but the habit which was in him previously is perfected, forasmuch as it has increased in extent; because the conclusions and demonstrations of one science are coordinate, and one flows from another.
Source: Summa Theologica (New Advent)