What you need is minimal 1 machine with a webserver that can host many virtual hosts under 1 or more IP numbers. On that host you run a database server of some

What you need is minimal 1 machine with a webserver that can host many virtual hosts under 1 or more IP numbers. On that host you run a database server of some kind, most used is MySQL (which can run many concurrent databases). Then each blog will run in a virtual host with a database connection to it's own database. If you use hppts, you need a wildcard ssl certificate that can accommodate all virtual hosts. This model can be up-scaled to separate machines for the webserver and database server, or even N number of machines running the webserver and N machines running the database server all behind a loadbalancer. All this even under different IP addresses, or usage of CDN services. In theory you can upscale this the size of Google services if you have deep pockets. Short answer, it's not the open source package but the infrastructure behind it, that defines the scale (of course efficient source code will help run things smoother).