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cf., Sent.I.D2.Q1.A4, Sent.I.D23.Q1.A4, QDePot.Q9.A5, CT.BookI.C50, 55, QVII.Q3.A1
The answer to this question is evident from what has come before. Since there are two processions in God (Q. 27), which form four real relations (Q. 28), there must be a number of persons. For, as we saw in the previous question (Q. 29), "person signifies something distinct existing in a certain nature....[thus] it signifies relation insofar as relation itself is toward the very thing related." (Sent.I.D23.Q1.A3)
From this, it is evident that the termini of these relations are persons and thus there is a plurality of persons in the Trinity.