Anyone that ran in the semi-final or final can win medals, up to six athletes. Two substitutions can be made between semi and final.
There are four runners per team who compete in a relay. However, teams always have alternates (back ups) in case of an injury or drop out.
http://nusports.cstv.com/sports/m-swim/spec-rel/081108aab.html This article seems to indicate that up to 8 swimmers, including those who are in the finals and semi-finals, will receive medals for the efforts of the final relay team.
butterfly, backstroke, breastroke, freestyle, IM, medley relay, freestyle relay
Jenny Thompson of the United States with 8 gold medals won. All of her gold medals came in relay events. She also won 3 silver and 1 bronze for a total of 12 Olympic medals.
This article from a small local newspaper says that all members whether they swim the prelims or the finals receive a medal: http://northbrook.patch.com/articles/north-shore-swimmer-wins-olympic-medal-in-relay
Jodie Henry of Australia won three gold medals at the 2004 Games in Athens ... 100 meter freestyle, 4x100 meter medley relay, and 4x100 meter freestyle relay.
Bjørn Dæhlie, Norway, has taken 6 Olympic Gold medals in individual competitions and 2 more as part of the Norwegian relay team. Tron Hummelvoll Oslo
Kriss Akabusi won three Olympic medals ... a silver in men's 4x400 meter relay at the 1984 Games in Los Angeles and a bronze in 400 meter hurdles and bronze in 4x400 meter relay at the 1992 Games in Barcelona. John Regis won two Olympic medals ... a silver in men's 4x100 meter relay at the 1988 Games in Seoul and bronze in 4x400 meter relay at the 1992 Games in Barcelona.
100m long jump relay 200m
Usaine Bolt won 3 gold in 2008. In 2012 he won 3 gold medals again. Both years the medals were won from 100m 200m and 4 by 100 relay.
No, preliminary track and field events act as a qualifying round for final rounds in the same event. Some events will even have multiple preliminary rounds. For example, at the Olympic 400 meter dash, there is first a preliminary round of 7 heats, from which the top three finishers in each heat automatically advance, then the 3 fastest times overall (for a total of 24 runners). Then there is a semifinal round, where the runners will race again in 3 heats with the top 2 runners automatically advancing, followed by the 2 fastest overall times (for a total of 8 runners). Then the final heat is run, with the top three finishers receiving medals, regardless of how they performed in previous rounds.