I have more than 20 paper wallets with very little bitcoin in them. How can I combine all of them together?
You should import all the private keys and not sweep them.
Wallets like blockchain.info has this function where it will import the private key. In that case the private key will become part of the new wallet.

Import all the private keys into a new wallet. Thats it ! During spending the wallet will combine inputs from your private keys to spend required amount.