Complex sentence: The stadium had a capacity of 20,000 spectators, but after reconstruction will be expanded to accommodate just under 73,730 spectators for Atlético Madrid matches of which all spectator seats will be covered by the new roof.
Split into simpler sentences: The stadium had a capacity of 20,000 spectators, but after reconstruction will be expanded to accommodate just under 73,000 spectators for Atlético Madrid matches. All seats will be covered by the new roof.