The U.S. Armed Forces are calling to active duty the Individual Ready Reserve (IRR). These are soldiers in the reserve who are not assigned to a unit (and are thus not paid), but are still serving out there remaining time (that they signed up for) in the reserves or active duty. Until their time is expired, they are liable to call up. Some are going to be called up. This is rarely done.
There are 118,000 men and women in the IRR. All have received their military training, served on active or reserve duty and been honorably discharged, but still have several years of military service remaining to serve. When you join the armed forces, you agree to a certain number of years of active service, plus some more in the inactive reserve. There are also soldiers who joined the reserves and ended up in the inactive reserve. This is because they moved and could find no reserve unit near where they lived, or none with a need for their particular skills. Those in the IRR are supposed to keep the military aware of their current address and health status, but this is often not done. The National Guard has a similar category, the Inactive National Guard (ING).
By law (10 USC 12302), the president can declare a partial mobilization (which he has done) that allows putting reserve and National Guard troops on active duty for up to 24 months. The president can mobilize up to a million personnel this way. To mobilize more personnel requires a full mobilization, and only Congress can do that, at the request of the president. With a full mobilization, an unlimited number troops can be called up for the duration of the war, plus six months. A full mobilization allows the use of the Standby Reserves (former active duty or reserve troops who volunteer to make themselves available for wartime service) or the Retired Reserve (all retired military personnel automatically pass into the Retired Reserve.) The Standby and Retired reserves provide over 100,000 well trained and experienced personnel. The number is vague because eligibility for active service depends on whether the reservists could pass the active duty physical exam.