Already there has been for you a sign in the two armies which met [in combat at Badr] - one fighting in the cause of Allāh and another of disbelievers. They saw them [to be] twice their [own] number by [their] eyesight.1 But Allāh supports with His victory whom He wills. Indeed in that is a lesson for those of vision.
Translation: Saheeh International (English)
1 The believers saw the disbelievers to be double their own number preceding the battle of Badr, while, in fact, they were three times their number.
There was a clear sign and lesson in the two groups that met to fight in the Battle of Badr. One group was made up of believers, and this was the Prophet (peace be upon him) and his Companions: they were fighting for Allah’s sake and in order to raise His Word against the word of those who disbelieve. The other group consisted of the disbelievers of Makkah, who had come out in pride and for show. The believers actually saw the disbelievers as being double their real number; but Allah helped His allies. Allah gives His support to whomever He wishes. In that there is a lesson and a warning for those of insight to realize that the help of Allah can come to the believers even though they may be few in number; and that defeat can come to those on falsehood even though they may be great in number.