This question graduated from the Sandbox for Proposed Questions
In my world, clothing and armor can be enchanted to grant protective effects. Clothing can be as thin as sheer silk, but still be strong enough to completely nullify a blow from a warhammer (or stronger).
My question is this. Given the information about my magic system below, what mechanic can I use to prevent a wealthy person from wearing a dozen layers of thin, enchanted clothing and becoming essentially invincible?
My Magic System
How is magic manifested?
Magic is energy in the world that casters take into themselves to become their own energy, which they then use for enchantments. However, a skilled magician can draw from ambient energy (has the least effect) or draw directly from their own energy (has the greatest effect), or between the two (moderates the effect).
What's the cost of using it?
Magicians pay nothing to cast an enchantment, but the amount of energy (ambient or personal) is limited. If there isn't enough energy to complete an enchantment, it will fail.
Can a person wear two magic rings on the same finger?
This question was asked in the Sandbox and I understand its point. I would prefer "yes" because I'd like people to have, for example, both a Ring of Fireball and a Ring of Shield on one finger. However, if a person stacks a bunch of Rings of Shield on all their fingers... that's actually what I'm trying to stop. So at the moment, I'm not sure what to do. (The outcome of this question will help me make that decision.)
What stops a magic user from casting a spell?
Running out of energy obviously stops the spell. Knocking them unconscious stops a spell. Disturbing them too much stops the spell (but the bar is set really high for most spells.) There are no "null-magic zones" where all spells are stopped no matter what, because even if you drain the ambient energy, the user will usually still have an internal storage of energy.
(Extra) Setting Flavor
The setting is a mix of (800s) East Asia and (1400s) Europe. Lots of tall, steep mountains with rugged terrain. Magic feels mystical rather than scientific. Medieval fantasy genre with Asian influence.