﴿ وَمَا جَعَلَهُ ٱللَّهُ إِلَّا بُشْرَىٰ لَكُمْ وَلِتَطْمَئِنَّ قُلُوبُكُم بِهِۦ ۗ وَمَا ٱلنَّصْرُ إِلَّا مِنْ عِندِ ٱللَّهِ ٱلْعَزِيزِ ٱلْحَكِيمِ ﴾

“And God ordained this [to be said by His Apostle] only as a glad tiding for you, and that your hearts should thereby be set at rest - since no succour can come from any save God, the Almighty, the Truly Wise -”

What God ordained that is of reinforcement was only as a good tiding to you of victory and that your hearts might be at peace that they might be at rest and not be terrified by the large number of the enemy as compared to your small number. Victory comes only from God the Mighty the Wise He gives it to whomever He will and victory comes not because of a large army.