It does send in batches with a size you decide, however it is using a form to carry the data on to the next page. So unless you have confidence your PHP installation can handle sending a huge amount of emails per batch (not recommended to bet on), you may want to consider installing my
"Automatic 'Send Email' Redirect" hack, which will automatically redirect to the next page, so you can play it safe and set batch amount to 500, then go watch some TV or summit