Complex sentence: The imbalance between the number of seats in the Security Council and the total number of member States became evident, and the only significant reform of the Security Council occurred in 1965: this included an increase in the non-permanent membership from six to 10 members.
Split into simpler sentences: The imbalance between the number of seats in the Security Council and the total number of member States became evident, and the only significant reform of the Security Council came to pass in 1965. The reform included an increase of the non-permanent membership from six to 10 members.