Even without knowing the standard issue side-arm (and many different departments were involved, so “standard issue” gets fuzzy) we can do a little speculative math.
377 rounds fired by 23 shooters is an average of just over 16 rounds each.
The standard issue side-arm for the US Army has a magazine capacity of 15. Even if every one of the shooters was armed with one these, and carried it with a round already chambered, somebody reloaded.
If just four of the shooters were armed with revolvers (6 shots), the average number or rounds fired by the rest jumps to over 18.
A powers distribution rule may likely apply here, something like the 80-20 rule. I would go so far as to speculate that a few “over-achievers” likely ripped through three clips (the one in the weapon at the outset, plus two spares) before all the shooting was over.
At nearly 50 rounds each, it wouldn’t take very many of the most trigger-happy to get the bullet count up to the obscene number recorded.
For the original question: “How many of them emptied their weapon then decided it was time to reload and keep shooting?”
At least a few, possibly many.
