Get length of json array in SQL Server 2016

Using a table instead of a variable:

SELECT count(priceLineLists.RoomTypeId)
FROM Mytable
CROSS APPLY OPENJSON (JsonDataCol, N'$.BasePriceLineList')
  WITH (
    RoomTypeId int)
      AS priceLineLists

The hypothetical SQL statement:

SELECT JSON_ARRLEN(JsonDataCol, '$.BasePriceline') FROM MyTable

Can be done by the actual statement:

SELECT (SELECT COUNT(*) FROM OPENJSON(JsonDataCol, '$.BasePriceline')) FROM MyTable