Caching. Everything can be cached, even just for a few seconds. I could write an essay about the different caching strategies, but it all depends on how comfortable you are with different tools and what you need, as opposed to what you want.
My tool of choice is
Redis, but you generally want to understand some of the fundamentals of HTTP when dealing with Varnish, there are lots of gotchas. Using VCL (Varnish Configuration Language) that you found somewhere on Google is going to bite you in the ass, mark my word. So going down that route, make sure you take the time to understand how Varnish works.
You can (in theory) has as many webpages as you have disk space and memory to serve. There isn't a definitive answer to how many, but more than one, sure. More than ten, probably. All depends on the resources available and activity.
Security? Keep things updated, don't use plugins that aren't vetted and regularly receives updates. For things like Wordpress this is absolutely crucial. Exploits to the "core" are very rare, and when they happen patches are always right around the corner. A (usually) safe option is to turn on unattended upgrades (for the distro of choice) and make it apply security patches automatically. You can also use tools like
needrestart to make sure that no processes are running older and vulnerable code.
When it comes to unattended upgrades you are trusting that someone know what they are doing. For most distros that is an assumption you can safely make when it comes to security. For regular updates it can be smart be more careful. But I doubt you'd have any issues.