Agreement is for 100M doses, with option for another 500M. The US government has previously invested in at least 4 other vaccine candidates, but this is the first investment in actual high volume manufacturing. This enables Pfizer+BioNTech to manufacture the vaccine in parallel with completing Phase 3 trials, and vaccine could be available by Dec 2020. Note that the previous stories I've seen indicate this would be a two does regimen, so 100M doses covers 50M people. No discussion on who would be first priority for receiving vaccine, but they did say that vaccine would be free to Americans.I've linked one story below, but there are several other stories on the same subject.

Pfizer Inc and German biotech firm BioNTech SE will get $1.95 billion from the U.S. government to produce and deliver 100 million doses of their COVID-19 vaccine candidate, the companies said on Wednesday.

The agreement allows the U.S. government to acquire an additional 500 million doses, the Department of Health and Human Services and the Department of Defense said.