Eight verses earlier (Num 11:17) has the same "some of the spirit" wording. Fr. George Leo Haydock's commentary on verse 17 says:
St. Augustine (q. 18,) reads "of the spirit which is on thee;" (Septuagint) referring it to the indivisible spirit of God, so that these ancients received what was sufficient for them, while Moses suffered no diminution. Thus one lamp communicates light to another, without being impaired. (Origen, hom. vi.) Theodoret (q. 18,) also adds, that a person confers baptism on thousands, and yet loses no part of the grace himself.
Haydock's reference to Augustine appears to be On the Trinity, Book 5, chapter 14:
And I will take of your spirit, and will put it upon them; that is, I will give to them of the Holy Spirit, which I have already given to you.
Cyril of Jerusalem, in Catechetical Lecture 16:25, says regarding Numbers 11:25:
The seventy Elders were chosen; And the Lord came down in a cloud, and took of the Spirit that was upon Moses, and put it upon the seventy Elders; not that the Spirit was divided, but that His grace was distributed in proportion to the vessels, and the capacity of the recipients. Now there were present sixty and eight, and they prophesied; but Eldad and Modad were not present: therefore that it might be shown that it was not Moses who bestowed the gift, but the Spirit who wrought, Eldad and Modad, who though called, had not as yet presented themselves, did also prophesy.
In other words, it's not that the Spirit was "apportioned" but rather that the power of the Spirit was conferred on more people.