I have over 150 certificates for client websites on one server. Does this mean that if 10 of those certificates renew on the same day, it will restart Apache 10 times, once after each certificate renewal? Or only once after all renewals are done?
I also noticed that some of the LE domain .conf files don’t have the renew_hook line at all. Why do some have it, but some don’t?
My web server is (include version): Apache 2.4.6
The operating system my web server runs on is (include version): CentOS 7.8
I can login to a root shell on my machine: Yes
I’m using a control panel to manage my site: No
The version of my client is: 1.7.0
Yes, though it's not a restart, it's a graceful reload. Traffic isn't interrupted.
To do it only once, you should use --post-hook rather than --deploy-hook.
Even if you had 150 certificates all with --post-hook "systemctl reload apache2", it would only do it once, at the very end. It's de-duplicating that way.
You can also drop an executable script in /etc/letsencrypt/renewal-hooks/post with the same effect.
If a --deploy-hook wasn't configured, the line is omitted.