Neither currencies nor coins are stored anywhere but at addresses out on the blockchain.
You may be talking about the secrets that give authority to send / spend those assets?
Seed Recovery phrases, passwords, PIN codes, private keys, master extended keys...
These are just data, text and/or numbers.
So of course millions of such data can be stored on a USB.
"Cold storage" just means, not needed for spending, kept away from any devices capable of connecting to a network, no wifi no bluetooth. Could be on a USB, or written on a paper or steel sheet kept in a safe deposit box.
Then you have the "wallet" data files or proprietary backup files as used by some client software, or a hardware wallet device.
In order to get good advice, please rephrase your question so we have more detail about your situation and just what you are trying to accomplish.
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