SPFieldLookup.AllowMultipleValues Property
Gets or sets a Boolean value that specifies whether multiple values can be used in the lookup field.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
Available in Sandboxed Solutions: Yes
Available in SharePoint Online
Property Value
Type: System.Booleantrue to specify that multiple values can be used in the field; otherwise, false. The default is false.