There really is no limit as to how many business someone can own and run. Witness today’s giant corporations, which often own 20 or 30 different companies. If they can do it, so can you, and for equally good a reason — sometimes it benefits you economically to have more than one company.
In the particular case of someone who wants to start a leasing company and lease trucks to another company that is a sole proprietorship, these two businesses must be kept financially and legally separate. They need to be set up as different companies, have different tax ID numbers, and file separate tax returns.