Background: Planning has been found to support behaviour change. How well planning interventions translate into behaviour change might vary according to the number of plans and baseline levels of behaviour. This study tested effects of the number of action plans and coping plans at two stages of change in physical activity, i.e. in motivated, less active persons (intenders) and active individuals (actors). Methods: Participants were 560 employees of a logistics service company who specified up to three action plans and coping plans or completed an active control intervention. Change in physical activity was measured 4 weeks later. Direct and indirect effects of baseline stage and number of action plans and coping plans were tested by variance analyses and regression procedures. Results: Intenders generated more action plans than actors, but stage groups did not differ regarding coping plans. Intervention effects on activity were strongest in intenders and those who specified more plans (two action plans, or three coping plans). The number of action plans mediated between baseline stage (intenders vs. actors) and changes in activity. Conclusions: The study underlines the effectiveness of action plans and coping plans, particularly in less active individuals. It further suggests identifying the optimum number of plans required to attain a satisfactory behaviour change.