According to a recent survey,

According to a recent survey, 48% of the approximately 30 million adults who remained uninsured at the end of 2014 were eligible for assistance under the Affordable Care Act, including 30% who were eligible for marketplace tax credits and 18% who were eligible for Medicaid.

Source: "New Kaiser Survey Finds Eleven Million Newly Insured Adults As of Mid-December, But Nearly Half Who Remained Uninsured One Year After Full Implementation of the Affordable Care Act Were Eligible for Medicaid or Marketplace Tax Credits," Kaiser Family Foundation, January 29, 2015, http://kff.org/health-reform/press-release/new-kaiser-survey-finds-eleven-million-newly-insured-adults-as-of-mid-december-but-nearly-half-who-remained-uninsured-one-year-after-full-implementation-of-the-affordable-care-act-were-eligible-for-m/