There really isnt an honest answer to this question.... the answer has lots of dependencies... such as what your willing to do and have available... multiple servers, load balancing, network etc...
a Dolphin site on a shared server is going to be quite limited, dedicated server will expand that number, then multiple servers, 1 for http, 1 for mysql, 1 for rms... then comes load balancing. Another main component will be what OS and what http server. then configurations will be key (getting mysql fine tuned, having RMS server fine tuned, etc.. for dolphin), killing off useless processes on a server, limiting the extension, because to be honest there is not 1 site out there that needs to use all extensions available, if you do then expect serious amount of servers and load balancing.
I had a huge vbulletin forum going, with all the bells and whistles going running linux with apache, that handled about 3k simultaneous members before bogging down, I then had the same site running lighttpd instead of apache capable of doing about 10k simultaneous.... I had 250k members about 70% were active daily, less than3% were active less than once per month. Let me also say vbulletin was a HOG of resources.
I assume dolphin will be also with even half the extensions being used so my dedicated server is about the baddest beast I could get, so give me a little time and will let ya know what the beast can handle before I add other servers....