If I want to generate for example 10 Ethereum addresses and I want to use Myetherwallet , I have to create 10 different keystore/.json  files and then use these files to access each address separately? Because that seems insanely annoying, considering that in principle it's advice against re-using the same address, I would have a bunch of these files to store, each with their own password...

Can I just get a file that can work as a seed that can spawn a bunch of different addresses? for example, you can use electrum, and save a file that you can load in electrum and have access to as many addresses as you want, not only 1 address per file. Is there any wallet that does this?