Of course we can only run 'what if' scenarios but there are really only 3 possible general categories of the effect of CCW carriers being there: The outcome was the same, the outcome was more positive (less dead), the outcome was worse (more dead).
Outcome was the same:
Not really worth discussing.
Outcome more positive:
These shooters were terrorists and likely a very different mentality than the mass shooters we have seen recently. ... ... If nothing else a few people shooting back, even if they never hit one of the terrorists, may have significantly slowed them down giving more time/less people killed before the police arrived.
Probably the most likely or maybe just the most hoped for out come.
Outcome more negative:
Really the only way for more people to be killed is if the CCW carrier started shooting people as well, likely by shooting at the terrorists and hitting other people instead. Probably everyone in the immediate vicinity of the terrorists were the first to die so the likelihood of a CCW carrier killing someone near them is pretty low. The most likely scenario would be where the CCW people missed and struck someone beyond the terrorist. This would have to happen to a lot of people to be significant in this event. While still tragic a single bystander killed by a CCW carriers bullet would have only made the situation 1.1% worse and that is assuming the bystander wouldn't have been killed by the terrorist anyway.
So, in my mind anyway, much more likely a few CCW's would have positively affected the situation rather than negatively.