Complex sentence: Sejm in the People's Republic of Poland started with 425 and at first the number was declared to represent 1 deputy per 60,000 citizens, but as the population grew in 1960 that connection was dropped from the constitution stabilizing the Sejm at 460 deputies.
Split into simpler sentences: Sejm in the People's Republic of Poland started with 425 members, and at first the number was declared to represent 1 deputy per 60,000 citizens. However, as the population grew in 1960, that connection was dropped from the constitution stabilizing the Sejm at 460 deputies.