Although you must control exactly one creature both when the ability would trigger and when it would resolve in order for the ability to create the Demon token, that creature doesn’t have to be the same one both times. For example, if the ability triggers, then you lose control of your one creature before it resolves, you can cast a creature with flash to make sure you control exactly one creature when the ability resolves.

If you control zero creatures or more than one creature at the beginning of your end step, the ability won’t trigger. If you control zero creatures or more than one creature when the ability resolves, it will do nothing.