Complex sentence: The size of the forces are not known exactly, though modern historians have agreed on estimates of around 10,000--15,000 on the English side and 20,000--25,000 on the French side.
Split into simpler sentences: The size of the forces are not known exactly. Some historians have agreed on estimates of around 10,000--15,000 on the English side and 20,000--25,000 on the French side.