109 generals sounds like a large number. It's a compelling metric!
But, I don't really know how many generals (and admirals) there really are in the military. I just know that the US military is kinda huge, so there's got to be a lot of them. I mean, the Pentagon alone is a massive building (the largest office in the world!) that I imagine being full of generals.
And then, I see in the letter that all of the signees are retired, which expands the total number of generals (and admirals) alive significantly.
I guess, I'm trying to ask a question about metrics, because I face this constantly. The absolute metric (>100 generals!) looks very strong, but the relative metric (nn% of all living generals and admirals active and retired) would possibly be underwhelming. It's no small feat to get a person with a real voice to sign a public letter like that so, the absolute number is big, but if the cause were of broad interest the relative number would be significant too....
On a minor bit of research I see there can only be 231 flag officers active at any time. I don't know how many are retired, but that does indicate the absolute number is a significant relative number. I still just struggle with the right way to report and digest metrics like this.