Since police are also there to combat murderers, gang members, drunk drivers, etc, your statement does not even begin to become valid unless police numbers are put against terrorists plus all those other groups. It's total nonsense.
No one is arguing that this is the only or the most important comparison that can be made. It's just one of many comparisons. That doesn't make it useless. I think a lot of people would be surprised to learn that cops are a much larger threat than terrorists, and I think the comparison can be used to make a much larger point about the utility of counterterrorism measures.
