Affordable housing levels stalled across the state over the last five years, and dropped sharply in many Boston suburbs, even as demand soars during a time of rapidly rising need. Despite a state law designed to ensure at least 10 percent of any community’s housing is priced below market, Massachusetts affordable housing stock fell to 9.1 percent this year, from 9.3 percent in 2006 after steady gains in the five years before that, according to state figures. Levels dropped or stayed the same in 64 communities in the Boston area. Housing advocates say the growing ranks of people who cannot afford to live in such an expensive region has led to dire shortages and that in some communities more than 100 people enter lotteries for a single subsidized apartment.