Measurement, trial, error and budget will help you determine how many marketing campaigns to run at once. You could have a Facebook marketing campaign, newspaper advertisements and a letterbox drop at the same time. If you are able to determine that all are successful, why not continue? The purpose of a marketing campaign is to boost awareness and raise sales so continue with each campaign that gets you results in these areas. If you find a campaign is underperforming, focus instead on the areas where you have success.