A cached file is typically generated after a user initially visits a page. This creates a delay for the very first user who views it.

The preload allows you to create the cache of all the post, page, category, homepage etc. periodically, which helps much in improving future page loads.

The preload feature starts to work after clearing cache. When the Preload feature calls the urls, the cache of urls are created automatically. When all the pages are cached, the preload stops working. When the cache is clear, it starts working again. The Preload runs every 5 minutes. If you want set a specific interval, you should take a look at the Manually Preload with Cron Jobs tutorial.

Note: The preload feature works with the WP_CRON system. Since there will be no action to trigger WP_CRON after the entire cache has been created, you need to set a cron job as you can see in this tutorial.

Number?

It depends on the power of the server. If you have a ordinary hosting package, you should not set more than 4-6. But if you have a VPS, you can set 10-12.

If you have a powerful dedicated server, you can increase the number as you wish. You need to add the following lines after <?php which exists at the top of wp-config.php file.

Restart After Completed

You can get more detail via the following link
http://www.wpfastestcache.com/features/restart-preload-after-completed/

Manually Speed Up Preload

You can get more detail via the following link
http://www.wpfastestcache.com/features/manually-preload-with-cron-jobs/

Re-OrderPreload

You can get more detail via the following link
https://www.wpfastestcache.com/features/re-order-preload/