Up to one million.

Via: Military.com:

President Donald Trump signed an order Friday allowing the U.S. military to bring members of the selected reserve and some former service members on active duty to support the government’s response to the novel coronavirus (COVID-19) outbreak.

The Army, Navy, Air Force, and Coast Guard can now order units and individual members of the Ready Reserve back on active duty for up to two years, according to the executive order. The order said a recall, if issued, could not result in having more than 1,000,000 members on active duty at any one time.

The order seems to apply mainly to members of the selected reserve, primarily comprised of service members who serve in a part-time capacity one weekend per month and two weeks per year.

Still, it also applies to those in the individual ready reserve, made up mainly of service members who served on active-duty and were discharged but still have a contractual obligation that subjects them to involuntary recall during a national emergency.