Where modern missiles would disable battleship turrets

Would modern missiles have to hit every main-gun barrel to disarm a battleship such as Yamato?

No; targeting the gun-to-mounting joints could disable or destroy an entire turret with one effective hit.

The question imagines using air-launched anti-ship weapons against Yamato's main battery. Hitting each 18-inch barrel directly is unnecessary. The useful aim point is the connection between barrel, gun mounting, and turret, where a successful strike can disrupt the whole mechanism. One well-placed hit per turret could therefore neutralize several guns by attacking their shared machinery rather than their thickest visible components.