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?
I can't say what is the best wallet to store your cryptocurrency right now but it seems that depends on the platform itself. Every wallet has different feature and that depends on your portfolio too. because not all wallet is supporting the various currency. You can use your USB flash drive to store your crypto but that's only like installed the offline wallet in your flash disk or just try to save your private key on there.
That's right. It depends on the platform. If all your altcoins are erc20 based, then I can suggest you to use meyetherwallet with ledger nano s. Which literally means hardware and online web together to make it safe and user friendly.