usually a standard adobe license on publish instance will support 200 concurrent users. dispatcher (aka apache) will not have any standard user load as it supports much higher number than publish instance.
Unless you are code is robust then it will support 200 concurrent users. Lets in you code you are not releasing the resources properly (like session or resourceResolver objects) then with in no time your requests will start fail even though it is not reaching 200 concurrent users.
As others already stated, it's hard to give any number for that, but in reality I haven't seen servers which are maxed out just by running the dispatcher. Normally the AEM publishs are maxed out much earlier 🙂
And regarding your second question: If you have a single dispatcher, it is one single point of failure. Not necessarily the only one, because in that case it's likely that you have a single publish instance, too.
I don't think there is any magic number where the dispatcher would fail and die if those many concurrent users access it at the same time. The performance of the dispatcher and that of your site mainly depends on the efficiency of the dispatcher caching. I know sites developed on AEM which are handling 1M+ page requests per day with only 2 dispatchers and one load balancer in place and without any CDN caching.
However, there is a chance of memory leaks happening on dispatchers(due to issues in patching or bugs in the version of apache itself), which over time results in one or both dispatchers dying and become unresponsive to further serve the pages.