I think you asked two questions.
How many concurrent users do I need to support for minimum market viability?
There is no universal answer for that. For a brand-new website in an niche market, 5 concurrent users may be enough. For the insurance enrollment website for the Affordable Care Act, the minimum number could be in hundreds of thousands.
It is possible that your product owner may be able to give you that number. More likely, here is what really happens: you deploy the site to production and watch whether it breaks. If it does, someone works on making it faster. That can be embarrassing for a national government or a major bank, but for a little start-up, it is a perfectly reasonable way to do things.
On the other hand, you really might want to find out about performance problems before you deploy to production.
Starting without any guidelines, how do I measure performance?
This is kind of like regression testing: you start by deciding on a set of paths through the system. If you are testing an existing site, you may be able to use the access logs to figure out which actions happen the most. If you are testing a new site, you don't have any users, so you have to guess.
Here are some criteria you can use:
- Choose paths that all or most users are likely to use every time they visit the site. For example, a YouTube user is likely to search for videos and view videos every time they use the site. They are unlikely to change their preferences every time.
- Choose paths that you expect to be server-intensive. If you know an action requires the server to do a lot of work, include. If you know an action requires locking a resource, include that too.
- Choose paths that need to be fast. We would like for every action to happen instantaneously, but some actions are more important to be fast than others.
Some test plans call for many users performing the same action at the same time, whereas others call for a random mix of actions. The former is easier to write than the latter, but in production, the actions will be random, so you should write your test that way too.