Complex sentence: Up to eight Guards have taken part, the number varying over the years: six in 1939, five in 1954, seven from 1963 to 1967, and then eight until the 1980s.
Split into simpler sentences: Up to eight Guards have taken part, the number varying over the years. There were six guards in 1939, five guards in 1954, seven from 1963 to 1967, and then eight guards up to the 1980s.