Depends upon where it is. Boston? San Francisco? Chicago? Easy. Oklahoma City, Salt Lake City, Dallas, not so much.
Adults with concealed carry permits by state: MA - 4.7% CA - .12%. IL - .34%. OK - 6.6%. UT - 9.7%. TX - 3.7%.
MA isn’t particularly good evidence for your case, having more CCW permits than TX. Some other states that you wouldn’t think of in this regard: IN - 10.8%. CT - 7.3%. PA - 8.7%. MS - 2.8%.
I’ve been unable to find any stats on how many people with permits actually carry at any given time, or if that varies by locations.
A CC weapon is a great deal better than nothing, but it’s a poor choice for engaging in combat against trained men with military weapons.
A Mumbai or Beslan type attack is generally not going to be stopped by CCW guys. It will take SWAT or Army, who will hopefully get there fast.