Complex sentence: The number of refugees who escaped through Portugal during the war has been estimated to range from a few hundred thousand to one million, impressive numbers considering the size of the country's population at that time (circa 6 million).
Split into simpler sentences: The number of refugees that has escaped through Portugal during the war has estimates that range from one a few hundred thousand to one million. Impressive numbers considering the size of the country's population at that time (circa 6 million).