It would be lucky if they managed to open over ten portals across his world in this way. The reason behind having hundred portals trying to open each time was the attempts of his enemies to open the portals fast.
Also they were trying to confuse him, making him mistake their intentions. If he fell into their trap, then he would commit the fatal mistake of thinning out his forces to cover such an astronomical number of portals.
But Arthur wasn't born yesterday. These dirty tricks might fool anyone else but him.
If he fell into their trap and thinned out his forces, then when the real attack would come, his forces would waste precious hours in redeployment.
Even this might put his nearby forces to the main enemy forces at risk of annihilation. He wouldn't desire such an outcome, so he didn't plan on thinning out his forces and spreading them across the large scale of areas where portals appeared.
