Complex sentence: Zoroastrianism is one of the world's oldest Monotheistic religions and has a history dating from 600 BC to perhaps more than 1000 BC. The total number of currently practicing Zoroastrians is unknown but estimates range from about 125,000 to over 300,000.
Split into simpler sentences: Zoroastrianism is one of the worlds oldest Monotheistic religion and has a history dating back at least from BC 600 to perhaps more than BC 1000. The entire number of Zoroastrians is unknown but estimates range from about 125,000 to over 300,000.