Over 80% of soldiers made it home
Source?

Some numbers from Great Britain : "Conscription put into uniform nearly every physically fit man, six million out of ten million eligible in Britain. Of these, about 750,000 lost their lives and 1,700,000 were wounded." Makes about 5,250,000 British survivors.
Source:
"World War I casualties", Wikipedia : http://en.wikipedia.org/wiki/World_War_I...
"World War I: Troop Statistics", Digital Survivors : http://www.digitalsurvivors.com/archives...
"Surviving veterans of World War I", Wikipedia : http://en.wikipedia.org/wiki/Surviving_v...