Hi there,
Where is the best to store 10 different altcoins? On physical device or online?
Can I store on physical device let's say my own USB flash drive?
Online web wallets more riskier than any type of bitcoin wallets. You will find the problem ate key shared wallet not shared wallets in online so you should always choose the private key shared one only.
For security reasons people have millions of USD worth of bitcoins in hardware wallet. That would be right choice since having cheap amount bitcoin you can have fund in desktop wallets or papar wallets that is enough.