CURLMOPT_MAX_HOST_CONNECTIONS (3) - Linux Man Pages
CURLMOPT_MAX_HOST_CONNECTIONS: set max number of connections to a single host
NAMECURLMOPT_MAX_HOST_CONNECTIONS - set max number of connections to a single host
DESCRIPTIONPass a long to indicate max. The set number will be used as the maximum amount of simultaneously open connections to a single host. For each new session to a host, libcurl will open a new connection up to the limit set by CURLMOPT_MAX_HOST_CONNECTIONS(3). When the limit is reached, the sessions will be pending until a connection becomes available. If CURLMOPT_PIPELINING(3) is enabled, libcurl will try to pipeline if the host is capable of it.
The default max value is 0, unlimited. However, for backwards compatibility, setting it to 0 when CURLMOPT_PIPELINING(3) is 1 will not be treated as unlimited. Instead it will open only 1 connection and try to pipeline on it.