The getblockcount RPC returns the height of the most-work fully-validated chain.
getblockcount none
*bitcoin-cli help getblockcount
getblockcount Returns the height of the most-work fully-validated chain. The genesis block has height 0. Result: n (numeric) The current block count Examples: > bitcoin-cli getblockcount > curl --user myusername --data-binary '{"jsonrpc": "1.0", "id": "curltest", "method": "getblockcount", "params": []}' -H 'content-type: text/plain;' http://127.0.0.1:8332/