Almost 12 million people signed up for 2018 health coverage under the Affordable Care Act, roughly the same number as last year despite a shortened enrollment period and a decision by the Trump administration to significantly reduce spending on outreach and advertising.

A total of 11.8 million people selected a plan or were automatically re-enrolled for 2018 coverage through the ACA’s federal and state exchanges, or marketplaces, according to a report Tuesday from the Centers for Medicare and Medicaid Services. About 12 million...