As I know, coinbase is the default account that stores ether when mining. Does a node only contain a coinbase? If there are many different users who have many accounts in one node, will there be many coinbase? If not, does it mean there are only one user (who may have one or more accounts) in one node?
Accounts can exist independently of nodes, they are comparable to a fingerprint you have with you. Most of the times, users run nodes and have their accounts/key files imported into it, and the first one is being defined as the coinbase. With the MetaMask plugin, for example, you can have accounts without a node, and thereby without a coinbase.