The oldest securely dated tin bronze artefact are found in the heart of the Balkans in Serbia. A tin bronze foil from the Pločnik  are dated to 4650 BC. The foil are not the only tin bronze artefact from the fifth millennium BC. 14 other artefacts from Serbia and Bulgaria are dated to before 4000 BC. The recent discoveries indicate that early tin bronze was more common than previously thought, and developed independently in Europe 1500 years before the first tin bronze alloys in the Near East. The production of complex tin bronzes lasted for c. 500 years in the Balkans. Shortly before the end of the fifth millennium BC, there are no longer evidence for production of tin bronze. This coincides with the collapse of large cultural complexes in the Balkans.Tin bronze would be reintroduced to the area again some 1500 years later.
Question: How many tin bronze artefacts from Servia and Bulgaria are dated to before 4000 BC?
Answer: 15