Adam Clarke on Psalms 68:17

The chariots of God are twenty thousand - רבתים אלפי שנאן ribbothayim alpey shinan, "two myriads of thousands doubled." Does not this mean simply forty thousand? A myriad is 10,000; two myriads, 20,000; these doubled, 40,000. Or thus: 10,000 + 10,000 + 20,000 = 40,000. The Targum says, "The chariots of God are two myriads; two thousand angels draw them; the majesty of God rests upon them in holiness on Mount Sinai." But what does this mean? We must die to know.