There's no hard limit to the number of spaces / communities you can create, but you'll definitely run into some things in the UI that you'll want to customize (for instance: the browse --> spaces drop down menu won't scale very well to 10,000 spaces). If you're at liberty to share, how exactly are you going to be using Clearspace with 10,000 spaces?
I am doing some tests at the moment and running into a few problems which I'm hoping is caused by the evaluation database (hypersonic). Switching to MySQL to be sure.
Can't really give too much details other than to say our client wanted a community application that also allowed personal work spaces. Clearspace seems a good fit and we are looking to assign each user (up to 10000) their own personal workspace for document storage, tagging and retrieval.