Is drupal bootstrap phases will be called for each page request or its only once on site load?
Since Drupal is a PHP application, there isn't a persistent process running on the server for it (unlike some other language frameworks).  This means that Drupal will bootstrap for each request it handles.  This can be a page load, a request to make an image derivative, an AJAX handler, a 404 page, and other things that aren't thought of as normal pages.  It also means that concurrent requests will result in concurrent PHP execution.

However, with the way it is designed, there are various levels of bootstrap, and different requests don't require the full bootstrap.  For example, a page request from a cold cache will require the full bootstrap.  When the page cache is enabled, and there is a hit, it will stop at DRUPAL_BOOTSTRAP_PAGE_CACHE.