public string OrganizationId {get; set;}

var portal = ArcGISPortalManager.Current.GetPortal(portalUri);
var owner = portal.GetSignOnUsername();
var portalInfo = await portal.GetPortalInfoAsync();

//1. Get all web maps
var query1 = PortalQueryParameters.CreateForItemsOfType(PortalItemType.WebMap);

//2. Get all web maps and map services - include user, organization
// and "usa" in the title
var query2 = PortalQueryParameters.CreateForItemsOfTypes(new List<PortalItemType>() {
  PortalItemType.WebMap, PortalItemType.MapService}, owner, "", "title:usa");
query2.OrganizationId = portalInfo.OrganizationId;

//retrieve in batches of up to a 100 each time
query2.Limit = 100;

//Loop until done
var portalItems = new List<PortalItem>();
while (query2 != null)
{
  //run the search
  PortalQueryResultSet<PortalItem> results = await portal.SearchForContentAsync(query2);
  portalItems.AddRange(results.Results);
  query2 = results.NextQueryParameters;
}

//process results
foreach (var pi in portalItems)
{
  //Do something with the portal items
}