If you're reading the manga, (covering chapters 122–131) takes us into the legendary U-20 Arc .
Across both mediums, "15" represents the moment where "playing safe" dies. Blue Lock, Volume 15 - Boston Public Library - OverDrive BLUELOCK 15
We see Isagi realize that he and Naruhaya are effectively "the same"—average players struggling to survive among monsters. To win, he doesn't just improve; he devours Naruhaya's off-the-ball movements to score a legendary goal. If you're reading the manga, (covering chapters 122–131)