3 warning shots.
At four Iranian ships carrying out a “high speed intercept.”
Now, I am not in the Navy, nor was I ever, nor do I play a Naval Officer on TV. However, the thing with big guns (and the 5’54” guns are big) is that they have a relatively low rate of fire. IOW, perhaps they wouldn’t be such a big deterrent. If’n I were the captain of that destroyer, I’d have been far more tempted to use the Phalanx CIWS for my warning shots - because that system is FAR more able to take out 4 fast boats quickly. That’s the system that you’d most likely use to actually defend the ship with, so why not use it for warning shots?
Because the bigger guns would be LOUDER to the distant Iranian ships, is why they were chosen, I bet.
Some of those fast Iranian boats carry 2 or more Silkworm anti-ship missiles ... which would be launched before the other fast boats made contact.
https://en.wikipedia.org/wiki/Silkworm_(missile)