It's a tricky question. You sort of have to freeze the timeline at some point. There's grassroots Cobra, with factions in most major cities (e.g. Springfields), then there's Empire Cobra, with legions of specialized Vipers (Air Viper, Eels, Eels v2, Snow Serpents, Ice Vipers, Toxo-Zombie-Vipers...), then there's Cobra post-Civil war which, while still Cobra, is fractured.

In my mind, Cobra has far more "civilian" members than actual soldiers. But then, I guess Crimson Guards under cover are still soldiers after all. Since their kids tote machine guns, might as well count them in with the soldiers. But the question still remains: is Cobra on the rise or diminishing? My Cobra collection is still in its beginnings (2 HISSes, a handfull of Troopers and very few specialized Vipers). But a Cobra at its height would easily be 500k-1mil including all it's low-grade troops (and families).