The race extension is complex, Although the main extensions maximum cardinality is 1, it defines sub extensions within it that allow for multiple races.  See the patient example:   resourceType: Patient id: example meta: profile: http://hl7.org/fhir/us/core/StructureDefinition/us-core-patient extension: url: http://hl7.org/fhir/StructureDefinition/instance-name valueString: Patient Example url: http://hl7.org/fhir/StructureDefinition/instance-description valueMarkdown: This is a patient example for the US Core Patient Profile . extension: url: http://hl7.org/fhir/us/core/StructureDefinition/us-core-race extension: url: ombCategory valueCoding: system: urn:oid:2.16.840.1.113883.6.238 code: 2106-3 display: White url: ombCategory valueCoding: system: urn:oid:2.16.840.1.113883.6.238 code: 1002-5 display: American Indian or Alaska Native url: ombCategory valueCoding: system: urn:oid:2.16.840.1.113883.6.238 code: 2028-9 display: Asian url: detailed valueCoding: system: urn:oid:2.16.840.1.113883.6.238 code: 1586-7 display: Shoshone url: detailed valueCoding: system: urn:oid:2.16.840.1.113883.6.238 code: 2036-2 display: Filipino url: text valueString: Mixed