I have an ETH keypair.
- Can I hold ERC20 tokens with this ETH keypair also?
- Can I hold several ERC20 tokens with one key pair?
You can hold both ETH and any amount of any Ethereum tokens in your account which is accessible with your private key.
It's up to the wallet you use whether it's able to display all the tokens or not. But even if you can't see them in the wallet client/app/whatever you still have them (assuming they were sent properly to your address).