A typical RAF Bomber Command 'tour' was 30 operations. Afterwards you'd typically get 6 months 'rest' at an Operation Training Unit or suchlike then you could volunteer for another tour, this one comprising 20 operations. The second tour usually was not with the original crew (maybe one or two members if it could be 'swung') or even in the original airplane type. Many Lanc pilots and Navigators flew a second tour on Mosquitos for example. If one was masochistic (or patriotic enough, depending on your definition) to do a 3rd tour that would comprise 30 'ops', then 20 etc. An aircraft bearing 50 'mission marks' on the fuselage indicates the airplane has completed 50 'ops' and bears no real reflection on the crew's experience although often crews would have a 'regular' plane they shared with other crews.