The largest customer site I've heard of in terms of number of pages is this one http://www.robertburnspoet.com/index.html which was created with Muse Public Beta 1 and appeared 10 days after Beta 1 was released. It's a site of all the poetry written by Robert Burns and has no images.
I've seen many customer files over 100 pages with the largest .muse file I've seen from a customer pushing 500Mb.
In general Muse is better suited to sites with under 100 pages. In most cases sites will a large number of pages would be more easily created and maintained using a CMS where a small set of templates are applied to data from a database to create a large number of pages on demand on the server. Since Muse is not yet integrated with the Business Catalyst CMS, it's not yet capable of this template+database approach to web page creation and maintenance.
What if it is being hosted elsewhere? Not on Business Catalyst. I have the need for a large number of files, but I'm exporting them, adding database code and application server code for data, and then I plan on publishing it. There realy won't be a demand on Business Catalyst.
However, I was toying around with the idea of creating one page, and then just creating multiple pages, but it seems we can't do this because each item on each individual page has individual IDs. Or is it possible (after exporting to HTML), to just duplicate a page and call it something else? Would I need to recreate the CSS with a new name? What about the IDs?