It really wasn't designed with that kinda scale in mind, but given that you can probably make it work...

I've been involved in a 20k+. It's not the funnest thing you'll ever do. If you have experience with concrete5 it's something you can make work, but you won't be doing this on a shared budget host, and you will have to make some compromises. That said I'm not sure this isn't true for most eCommerce options. Also this doesn't involve any variations.

There are some things with the concrete5 page object and when you get tens of thousands of pages under the same page you need to realize that you can't do certain things or need to mitigate against others.