I want to try to use my raspberry to host a standard wordpress personal blog. Yes I know this could sound a crazy idea but I'm wondering roughly what range of users could handle assuming a standard Wordpress installation with default theme (no complex themes and extra plugins that could increase the resource usage).
Is there someone that has experience on this kind of Rapberry use case that cloud give me a clue about this matter?
If you test a web server locally on a Pi 3 serving up cached static content, it should be able to do at least hundreds of pages per second. By "static content" I mean pages whose content is not changing dynamically to suit different users or fluctuating data. By "cached" I mean that there is a limited set of pages being served over and over, such that the OS can cache them in free RAM rather than having to read them from storage every time. This means you need to make sure the system does have some free RAM -- on a Pi 3 at least 25% of the total available, i.e. ~250 MB.
By locally I mean on the Pi itself. This represents the maximum performance of the server, and again, it should be hundreds (or perhaps thousands) of small (< 4 kB) pages per second. Richer content (images and such) obviously require more overhead although this should be mitigated by caching on the client side.
In reality the server, and the CPU and RAM resources at its disposal, are unlikely to be the bottleneck. Although 4 MB/s (1000 * 4 kB) is well within the Pi 3B+'s ethernet potential (~35 MB/s), in reality I think you will find, especially if these are small bursts to many different clients, the network connection will not be able to keep up with the server application. In other words, if a local test maxes out at least one processor (or better yet, three), an actual network test will actually show the CPU being under utilized and the number of pages served less than the local test. This indicates that the bottleneck is the network connection.
For April 1st 2016 I migrated the main Raspberry Pi Wordpress blog on to 8 Pi 3Bs (not the + model). https://www.raspberrypi.org/blog/the-little-computer-that-could/
We were unable to migrate the (large) database backing the site onto the Pi 3 as it didn't have enough RAM to perform properly, the Raspberry Pi database is much larger than a typical Wordpress blog. In normal usage Wordpress Supercache was used to accelerate the Raspberry Pi blog so that the majority of pages are delivered as static HTML - around 90%. On a typical day the Raspberry Pi blog sees around half a million users which was adequately served from 8 Pi3Bs, our graphs suggest we'd have got away with just two.
For the launch of the Raspberry Pi 3, 5% of the page requests were served directly from a Pi3, approximately 1.2m. Launch day has a higher cache hit ratio than normal - vast numbers of people arrive, look at the launch page and leave so this is an over-estimate for a normal site. https://blog.mythic-beasts.com/2016/02/29/hosting-the-raspberry-pi-3-launch-on-a-raspberry-pi-3/
For a simple Wordpress blog, Wordpress Supercache installed with around a 75% hit ratio you should see page rendering in around 1s. You've four cores so you can render roughly four pages per second, and you get 4x that for free due to the caching. That works out at around 1.2m page requests in a day at perfect efficiency, allowing for a typical usage curve you'll get between 300k and 500k. If you want to try it for real Mythic Beasts run a Raspberry Pi cloud, https://www.mythic-beasts.com/order/rpi
Ben Nuttall runs PiWheels on this Raspberry Pi cloud, all of the downloads are done by a single Raspberry Pi 3B, which typically downloads over 20,000 packages per day.
