It wouldn't take 20 bombs.
It wouldn't take 2 bombs.
It would just take one. Because after that all it would take is an anonymous web post or call to the news station and nobody who works downtown would show up the next morning. Economic collapse. Not down turn, not recession, great depression.
This country isn't as peaceful and unified as we pretend it is. The prosperity to pay for bread and circuses and lots of men in blue keeps the lid on. It didn't take New Orleans 24 hours to revert to behaving like Mogadishu when those things were interrupted.
What's going to keep the lid on when we can't afford those anymore? We'll find out. But it doesn't take much imagination to see it'll more closely parallel 1930s Germany or 1990s Russia than anything we've seen here.
(sounds silly I know, but I would feel safer in numbers)