Server sizing is a deeply personal and technical thing, and generally requires more data than can be put into a ServerFault post. This is why they nearly always get closed. There are some things people know we need to know, and they're pretty good about including them:
- Concurrent connection count
- Total number of users
- Some idea as to how much data will be used
- Technology frameworks in use
Good, but not enough to size anything on. The one variable that is very, very hard to evaluate with no access to the system (which is what all of us answerers will be doing) is code performance. In the grand algebra of how much server do you need, how efficient your code is in handling each transaction has deep impact. Benchmarking is the only real way to get this data.
And for sizing questions involving off-the-shelf software where code-efficiency could reasonably be viewed as common knowledge to the right subset of people, the other variable, how your users use the system has significant impact as well.
These two items:
- Code efficiency
- User usage patterns
Are things we can't judge for you. Unless you have that, we can't help you size something beyond generic guidance. That may be what you're looking for, but if we keep going down that path we'll develop a stable of 'canonical answers' that we'll close-as-dupe questions to.
A better question to ask than, "how much server do I need," is, "How can I figure out how much server I need." This second question allows us to answer with techniques for figuring it out, rather than hard values we're wild-ass-guessing on. Specific technology frameworks have their own ways of determining performance, and you may learn quite a bit about benchmarking in the process.