4.5 million per week was one number I came across. If I remember correctly it was based on the the number of weekly vaccines allocated to states. Since other countries are getting vaccines as well, the actual amount manufactured is probably higher.
AAQ, I don’t know who said they were given no plans. I did see a clip of fauci saying the Biden administration is not starting from scratch. Your original question was why is 100m doses in 100 days ambitious, which is entirely different.
Maybe a good analogy is if you have a weekly magazine subscription that piles up for a few weeks. If you sit down and look through 4 back issues one week, it doesn’t mean you can continue reading 4 issues of that magazine per week going forward because you’re still only getting 1 new issue every week. To maintain that pace, you’d have to either get the magazine to publish more frequently, or subscribe to additional magazines. It’s more complicated than just maintaining the current plan.
That said, I’m sure some aspect of this is to set expectations that you can beat. But it does appear to be more complex than just continuing the plan rolled out by the previous administration.