you start with three units (assuming you're not Russia). You can only control 1 unit yourself. You can choose which. With the other 2 units you can either hold or give another power control. They would still be your units, but the orders are given by other powers. As long you have up to N units that are under orders (you can render a unit 'inactive' so you're not forced to let your enemies control your units. It just holds then.), with N being the amount of powers alive, every country, including yours, can only control one of your units. With N+1 active units, you can control 2 units yourself. Up to N*2, you can give other countries over up to 2 of your units as well. It goes on like that. This requires you to have good alliances, because you need allies to control your units, unless you want them inactive or rogue.
Be sure to ask any questions.