I remember hearing this question and I've asked it a few times throughout my career.
The answer to "how many trucks are on a post?" is one. The truck is the gold ball on the top of the flag pole. There were also mixed stories about there being a razor and a bullet inside the truck. That was for in the event of the post being over run, the post commander would use the razor to destroy the flag and the bullet for his brain.
The razor and bullet story seems far fetched and unlikely but entertaining nonetheless.
Three Saints - more help with the board under the "projects" page