Tesla is expanding its “Full Self-Driving” (FSD) beta, CEO Elon Musk said, doubling its size with a new software update, and eventually increasing the number of participants tenfold.

“Still be careful, but it’s getting mature,” Musk tweeted Friday. He said Saturday that Tesla would add a “download beta” button to Tesla displays in the coming days “due to high levels of demand.”

Tesla first introduced the FSD beta in October to vehicle owners in its Early Access Program. At the time, Musk said the company was handling the software update “very cautiously.” Drivers still are expected to keep their hands on the steering wheel and should be prepared to assume control of their Tesla at any time. Tesla uses the data from the FSB beta to improve performance, and according to Electrek. Musk said last month there were about 1,000 vehicles in the beta.

Musk noted that in Tesla’s case, the term “beta is used to reduce complacency in usage and set expectations appropriately. All software is first tested by Tesla simulation and QA drive teams.”

He added that availability of the FSD beta will vary by region, due to what he called regulatory approval delays and Tesla’s own internal development and testing. Tesla’s director of AI tweeted Saturday that Tesla owners interested in participating in the beta should email earlyaccess@tesla.com, which he says the company is using to coordinate the program