Barring unexpected developments, after Barack Obama is inaugurated later this month, there will be four living former presidents: Jimmy Carter, George H.W. Bush, Bill Clinton and George W. Bush.
The number of presidents alive at any one time (including the incumbent and former presidents) is tracked on Wikipedia.
The highest number of former presidents alive at one time is five. The most recent period during which there were five living former presidents lasted from George W. Bush's first inauguration on January 20, 2001, to Ronald Reagan's death on June 5, 2004.
The number has generally been high in recent years. However, in 1973 and 1974, there was a period during which there were no living former presidents. That period began when Lyndon Johnson died on January 22, 1973, and ended when Richard Nixon resigned on August 9, 1974. The number then consistently rose, until Nixon died on April 22, 1994.
Theoretically, with life expectancy having increased for the general population, the number of living presidents should stay above the historical average.
More to come, regarding former presidents, in future posts.