The chariots of God are tens of thousands and thousands of thousands;
The Lord [has come] from Sinai into his sanctuary. (Ps 68:17).
The above translation is from NIV, and it suggests that some emendation of the text may have been done in the part, “has come.” The following is the translation of KJV, which is more preferable:
The chariots of God are twenty thousand, even thousands of angels:
The Lord is among them, as in Sinai, in the holy place.
Do not worry about the differences in the first line. The important thing is to note that the word “chariot” (Hebrew reheb) is translated in plural. The word in Hebrew is in singular, but because of the meaning of the predicate it is translated in plural. In fact, this is a collective noun—singular in form, but plural in meaning. Or, we can translate it “chariotry” rather than “a chariot.” The second line of the verse definitely refers to the event in Exod 19:
Mount Sinai was covered with smoke, because the Lord descended on it in fire. The smoke billowed up from it like smoke from a furnace, the whole mountain trembled violently, and the sound of the trumpet grew louder and louder. Then Moses spoke and the voice of God answered him (Exod 19:18-19).
Keep in mind that “the sound of trumpet” is often used in war as a sign for the move of an army. The idea is that God appeared accompanied by his chariots—tens of thousands of them—“in fire.” In fact, the same chariots of fire fought against the Egyptian chariots, and divided the water so that the people of Israel could cross the sea walking on the dry ground.
It reminds us of the words Jesus spoke at Gethsemane to Peter who struck one of the servants of the high priest by sword:
Put your sword back in its place, for all who draw the sword will die by the sword. Do you think I cannot call on my Father, and he will at once put at my disposal more than twelve legions of angels (Mat 26:52-53)?
There are some interesting stories in 2 Kings regarding God’s chariots. One time, Elisha said to his servant when his city was surrounded by a vast army of king Aram with horses and chariots:
Don't be afraid. Those who are with us are more than those who are with them (2 Kgs 6:16).
Elisha prayed that the eyes of his servant would be opened. The Bible says:
Then the Lord opened the servant's eyes, and he looked and saw the hills full of horses and chariots of fire all around Elisha (v. 17).
Here, the Hebrew word for “chariots” is reheb in singular, but correctly translated in plural. This leads us to question about the chariot—usually translated as singular—that Elisha saw when Elijah was taken into heaven:
As they were walking along and talking together, suddenly a chariot of fire and horses of fire appeared and separated the two of them, and Elijah went up to heaven in a whirlwind. Elisha saw this and cried out, "My father! My father! The chariots and horsemen of Israel!" And Elisha saw him no more. Then he took hold of his own clothes and tore them apart (2:11-12).