A few hundred. By balloons launched from Japanese subs off coast of Northwest US. No accurate record available because the discoveries were strictly kept as secrets.

http://news.nationalgeographic.com/content/dam/news/photos/0... secrets in 1943 perhaps ... not now though.

Not a secret now of course. I'm saying because it was kept secret at that time, not good records were kept.

There also was (at least?) one bomb thrown from an airplane (launched from a submarine). http://arcweb.sos.state.or.us/pages/exhibits/ww2/threat/bomb...:

"On September 9, 1942 Japanese pilot Nobuo Fujita catapulted from the I-25 near the coast of southern Oregon aboard a seaplane and headed east toward Mt. Emily. His mission was to drop an incendiary (fire) bomb on the thick forest and cause a massive fire that would shock Americans and divert resources from fighting the war."

And I guess thousands, including live ones, were dropped in training exercises and during tests.

Japanese balloon bombs were just in the news this week. A Utah sheriff had a bit of fun with one in 1945.

http://www.ksl.com/?nid=148&sid=34756436

Also a discussion on HN about Japanese balloon bombs a few months back: https://news.ycombinator.com/item?id=8922927

Dutch Harbor?

Are you assuming Pearl Harbor is pre-war?