Ethereum is not your typical PKI infrastructure.
The public key is derived in a deterministic way from the private key - same private key always leads to the same public key. The public key is not "validated" in that sense by any authority - it's always valid. The private key can be chosen with whatever means.
Furthermore, the public key is used to generate a public address in a deterministic way.
Nothing stops a user from generating multiple public keys. As they are not validated in any way, there is no overhead in generating "too many" public keys. The system doesn't even know which public keys have been generated - it only has records of which public addresses have been used for transactions. So nothing also stops you from sending a transaction to a random address - you have no way of knowing (without checking whether has issued transactions from that address) whether someone possesses the corresponding private key or not. If you choose the address at random, it's highly unlikely that someone has a private key which results in that address.
There are plenty of more detailed articles online about the generation of private key / public key / public address. Here's the first hit from google: https://medium.freecodecamp.org/how-to-create-an-ethereum-wallet-address-from-a-private-key-ae72b0eee27b