Housing Choice Vouchers five million households cbpp.org extremely low income Housing Choice Vouchers Section 8 Housing Choice Voucher Program serves over five million people Housing Choice Vouchers Housing Choice Vouchers housing vouchers "Policy Basics: The Housing Choice Voucher Program" five million households Section 8 Housing Choice Voucher Program Section 8 Housing Choice Voucher Program more than 2.2 million U.S. families plurality notes Section 8 Housing Choice Voucher Program Section 8 Housing Choice Voucher Program important vouchers