The number of supported members on your site will largely depend on your server.
To be more specific, Intel(R) Pentium(R) D CPU 3.40GHz 2 cores 3GB RAM CentOS release 4.7 will hold up to 90-100 thousand users.
Xeon E3-1240 3.3 GHz (4 cores/8 threads), Intel S1200BTL, 8GB RAM, 2×1000 GB HDD SATA, 2xGigabit LAN should hold up to a total of 500 thousand users, among them 100-150 passive users online and 10-20 active users online.
By active users, we mean people who perform search simultaneously. Passive users can view profiles, chat, read blogs, and engage in other site activities that are not concurrent.
More powerful servers can hold even more users than that.
It is also possible to optimize the software and server configuration for better results with the help of the PG Experts team (firstname.lastname@example.org).