Like anything i would guess XF would need some tweaking but then i don't know of any forum solution that wouldn't with that type of user base.. I doubt it would be anything serious. And as Steven mentioned it would be more on the hardware then the software.. and umm yeah you'd either need to be extremely dedicated or hire out professionals to maintain a site that large.
The question is not the number of users - the question is how many are on the forum at one time, and how many total posts there are!
If you look around at various really big forums, you rarely see more than 1000-1500 users online at one time (during the measured period, usually 10 or 15 minutes).
IMHO, a forum with up to 2000 people online at once using it in typical ways (maybe 1/2 are guests just reading and 1/2 are reading and replying and asking questions) would be able to be hosted on a single dedicated server with this (and other) forum software. You would not want to cheap out on your server! I'd say two 4-core CPU's and 8G of RAM would do the job.......
In order to get a better idea, you should know how many page views you expect in a busy hour. Traffic tends to come in bursts, with busy times often being sunday and monday evenings, etc.....at certain hours.
"It depends" ends up being the answer to a lot of these questions.....but for a simple guide, you should know about the page views. One would assume that if you plan to have one of the largest online communities in the world, you already have something like it? If not, you might be wishful thinking.......
Oh, also, you can use Amazon cloud services for this now....which allows you to scale up as big as you like (in theory anyway).....if you truly think you are going to build up that kind of audience. This effectively gives you access to almost unlimited computing power......
I don't see where XF would make a vast difference if a lot of your calls are to MySQL. Sure, things can be tuned better ....but VB 3 was written by the same team!
I am not a inside tech person by any means, but MySQL and many apps which use it are probably not designed for extremely large sites - of course, large being relative! The kind of traffic you mention is probably experienced by only the top 100 forums in the world (if that!)
Your number of users online seems doable.......I think a lot of this depends on how the particular Server and CPU setup distributes tasks (threads?) to the CPU(s).
As an example, I am using an older forum and CMS on my site and just upgraded to a XEON mid-level server.....actually pretty low level. But my mid-sized and relatively inefficient forum and cms with 800,000 posts, 23K members and 700 members online at the same time.....generating 100,000 page views or more a day, is not even starting to hit a server load of 1. Being a 4 core CPU, I'd think the server would not start sweating until the load hit over 4 - my estimate (based on nothing at all) is that with about 1200 users at one time we'd hit that.
Now, let's say I had a more efficient forum SW (XF, etc.) and two servers (db and httpd) and each was pretty upscale - like dual Xeons each and 8G or 16G or RAM each......
It's hard for me to imagine that such a setup would not handle thousands of users online at one time....
However, I think we mere mortals deal with too many variables to actually say without testing
Well, you can write efficient queries or not. Think about where conditions with wildcards oder unneeded joins and subqueries (due to bad database design). SQL Optimization is a tricky task and needs lots of experience and knowledge..
currently my forums runs well with vb3 and 500 Users online on a single server with about 1gb ram. But with that load the cpu and harddrives are quite busy. I doubt that my machine could serve more than 1000 Users online at once. But then is the question how about buying new hardware or disabling some query-dependent features of the forum.
I run a forum with the following stats:
Average of users online: 4,000
My specs are as follows:
Processor: AMD phenom || X6 1055T 6-core Processor
We use Litespeed webserver and MyBB 1.6 and rarely get any problems, But we do plan to upgrade to XenForo when a stable version is out.
P.S We did upgrade temporary to XenForo, I borrowed our Partners XF license, I upgraded it on a different server and we upgraded from MyBB to Vbulletin then to XF and things was good, It was mainly a test to see how things performed, So yeah XenForo can handle a fair bit of users, Although we did not test it on a production forum, Just with our users, threads and posts
This forum is quite busy, runs on a single server, and is lightning fast: http://www.backyardchickens.com/forum/
They typically have 1000 members on at one time (5 million plus posts)....I think they use myBB,.......whatever it is, it seems quite fast and uses only a few queries (compared to my forum) to render each page.
Just another example of possibilities.
FYI, one test I read said that the alternatives to apache only really worked better serving static or default setups......but that for the php dynamic stuff, all was somewhat equal.....given a well tuned Apache 2.2 setup.
Still, it never hurts to have more than enough speed and tuning!
We did use Apache in the past but we decided to go LiteSpeed Because we though it could handle a lot more than what Apache could as our forum grew larger each day and the server was under a lot of stress, We reilized then that our dedi was the problem to we upgraded to what we are at today, Although We have not yet tested Apache on our new server yet, We will defiantly upgrade to XF soon but not yet because we are happy with MyBB because it offers a lot with the plugins, But yeah we are going to purchase a license soon.
When big events happen (ex/ product release or announcement), we have as many as 18k users online at once. Our current record according to the stats page in the AdminCP is 18576 users online at once. Note: we hit "only" that many users because the servers couldn't handle any more load.