According to 2 Chron 17:13-19, Jehoshaphat had an army of 1,160,000 men stationed in Jerusalem.  This number is over and above those mentioned in v2.

This number suggests that there were more Judahites fighting for King Jehoshaphat than there were Russian soliders in the Battle of Kursk (the largest battle in military history).

Various attempts have been made to make sense of this number.

(a) Some take the number literally

JFB appear to accept the number without hesitation, remarking:

‘No monarch, since the time of Solomon, equalled Jehoshaphat in the extent of his revenue, in the strength of his fortifications, and in the number of his troops.’

The contributor to HSB (Walter Kaiser) notes that ‘many scholars’ have thought these numbers to be ‘excessive’.  But:

‘there are no other comparative figures with which to judge the authenticity of this number except what moderns regard as “excessive.”’

This, I think, is making light of the problem.

Ellicot, while hesitant to take the numbers at face value, suggests that what is written here is

‘simply an estimate of the total male population liable to be called out for the national defence.’

The text, however, specifies that this huge army was actually stationed in Jerusalem (and not dispersed around the country).

(b) Others think that the text is simply wrong

This appears to be the opinion of Keil & Delitzsch:

‘These numbers are clearly too large, and bear no proportion to the result of the numbering of the people capable of bearing arms under David, when there were in Judah only 500,000 or 470,000 men (cf. 1 Chron. 21:5 with 2 Sam. 24:5); yet the sums of the single divisions appear duly proportioned,—a fact which renders it more difficult to believe that these exaggerated numbers are the result of orthographical errors.’

(c) Still others regard the number as reflecting the ‘semi-barbaric’ nature of the times

To the modern child (it is argued) one hundred is just as large a number as one million.  Such biblical numbers are not the result of deception, but of idealisation.  (See discussion in A.H. Strong, Systematic Theology, Part 3, ch. 3, and the quotes therein).

(d) Another suggestion is that the text has become corrupted in some way

Barnes:

‘It is probable that the original numbers have been lost, and that the loss was suppplied by a scribe, who took 2 Chronicles 14:8 as his basis.’

B.C. Birch (ISBE) appeals to the disupted translation of the Heb ʾelep̱ (“thousand”), which may originally have meant “tribe,” “clan,” or “group”.

Pratt (Mentor Commentary) agrees that the underlying word may refer to ‘unit’ or ‘chief’, yielding much smaller (but still significant) numbers.

Selman:

‘As elsewhere, thousand is probably best understood as a military unit, though these figures are still unusually high (cf. 14:8; 25:5–6; 26:12–13).’

(e) It may be that the numbers as symbolic in some way.

It is noteworthy the Jehoshaphat’ s 1,160,000 is exactly double the number who were taking up arms under Asa.  And Asa’s 580,000 is the sum of Abijah’s 400,000 and Rehoboam’s 180,000.

Dillard (WBC) suggests that, however the numerical problem is solved, large armies are, for the Chronicler, indices of divine favour resting upon the king.

Lange:

‘The…780,000 Jews and 380,000 Benjamites can scarcely be accepted as literally true. Their near approach to the numbers resulting from the census taken by David (1 Chron. 21:5) seems intended to convey the idea that the kingdom of Judah alone had under Jehoshaphat, the alter David, attained a strength which almost matched the power of the twelve still united tribes under the first David (1,100,000 Israelites and 470,000 Jews), that Judah by itself alone had now developed a number and power which surpassed that of the northern tribes at that earlier period.’

I am inclined to accept some version of this last explanation, although precision and certainty cannot be achieved with our present level of knowledge.