The default ring size for current Monero transactions is 16.
A Monero ring is the group of possible signers attached to a transaction input. It contains the real output being spent plus 15 decoy outputs selected from the blockchain. Ring signatures let a transaction prove that one member authorized the spend without revealing which member was the genuine signer.
The number has changed over Monero’s history. Protocol version 0.13 made ring size 11, replacing earlier flexibility with a uniform requirement. The network upgrade associated with protocol version 0.18, activated in August 2022, raised the ring size from 11 to 16.
A frequent mix-up is answering 15 because that is the number of decoys, not the total ring size. Another is remembering the older value of 11. Ring size also describes possible signers for each input; it is not the number of inputs, outputs, or signatures in the entire transaction.