For Q1 – One for each path / site. Each site has its own cron (jobs) internally. So, it is possible to fine-tune the frequency. For example, a low-traffic site may not have lots of cron jobs. So, the cron could be executed less frequently than a site that requires frequent updates.
For Q2 – You may use GET method. Also, instead of cron command, we may use a simple wget too. The query string “doing_wp_cron” is optional. A simple way to trigger WPCron is…
wget http://example.com/wp-cron.php
You may know about how everything fits together in the official developer documentation for WP Cron.