Complex sentence: The United Progressive Alliance (of which INC is the biggest member) fielded 541 candidates for the Lok Sabha elections; 465 belonged to INC and the rest were fielded by the other members of the UPA.
Split into simpler sentences: The United Progressive Alliance of which Congress is the biggest member had fielded 541 candidates for the Lok Sabha elections. Of these 540 candidates 465 were of Congress and the rest had been fielded by the other members of the UPA.