The getblockcount RPC returns the number of blocks in the local best block chain.
getblockcount none
*bitcoin-cli help getblockcount
getblockcount Returns the number of blocks in the longest blockchain. 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/