Complex sentence: AIMCO is the United States's largest owner and operator of apartment communities, with approximately 1,475 communities comprised of nearly 260,000 units in 47 states, the District of Columbia and Puerto Rico.
Split into simpler sentences: AIMCO is the US's largest owner and operator of apartment communities, with approximately 1,475 communities that include approximately 260,000 units. They have apartment communities in 47 states, the District of Columbia and Puerto Rico.