Later in the story, Pace identifies these as "476,000 applications that have been started," not necessarily completed (the Obama administration describes them differently - more on at in an update at the bottom of this post). Nowhere does the story get into enrollment; those figures, Pace writes, won't be available until mid-November.
These numbers tell us that about half-million people are interested in shopping on the marketplaces. But they don't tell us much about how many people will actually sign up. There are two significant steps that stand between filling out an application and purchasing a health plan: Finding out what program you're eligible for, and actually selecting an insurance plan.
Measuring enrollment is a difficult proposition. Most health insurance plans don't count shoppers as enrolled until they've actually submitted a check for their first month's premium. That means they're entitled to start using the benefits of that health plan come Jan. 1, when any coverage purchased on the marketplace right now starts.
This is actually a difficult metric for the White House to measure, since it's the insurance plans - not the federal government - that collect those payments. While the marketplaces send the insurance plans a nightly file showing who signed up for their products, its not the exchange's function to collect payment from those individuals.
Still, there are slightly more robust metrics that individual states have used to measure interest. Many are reporting the number of applications completed, showing individuals who have gotten further in the process.
Some of these applications will not convert into enrollments, if shoppers decide they don't want to actually pay for the product. Still, these are the cases where people have made it a little bit further in the shopping process, and might make a slightly better proxy for estimating how many people want to enroll in coverage.
Update: While the Associated Press article describes the 476,000 applications as ones that have been "started," an administration official said that they are actually submitted applications. This post has been updated to reflect that change.