Complex sentence: The numbers of the fighting parties are unknown, apart from folk tales of the Ursidae cavalry but the military readiness, ability to field soldiers and the rank of opponents indicate that there could not have been more than 4,000 men on each side.
Split into simpler sentences: The numbers of the fighting parties are unknown, apart from folk tales of the Ursidae cavalry. However, the military readiness, ability to field soldiers and the rank of opponents indicate that there could not have been more than 4,000 men on each side.