Runes are used to enhance the human body's capabilities. They are inscribed onto the skin with through a ritual and work by the individual accessing the mana inside themselves. This mana is forced into the runes in order to activate them, creating the effect. Different runes have various uses. Super strength or speed, additional toughness, eagle eyesight, etc. Are some of the powers they grant you. There are even runes that add specific abilities not found in the body, such as generating a sword or shield, summoning a fireball, or turning invisible for a period of time. These runes fade away after repeated use, and must be re-applied at some point. They last for a period of time related to quality and skill of the maker.
An individual should only be able to use one type at a time. If they want to change their runic abilities, their current runes needs to be removed and replaced with a new one. I need a reason for why people cannot use different kinds of runes at once for a hodgepodge of various abilities, even though they all work the same way. How can I accomplish this?