Ok that makes sense. Some of these third party modules don't seem to behave well across portals. And if they only use the core modules then I can buy that.

Regarding limitations, there are no known limitations. From time to time we see scenario's where a particular setup highlights an area of weakness e.g. the user who had 8000 portals had a lot of problems. When we looked at his setup, the issue was not the number of portals, but in fact the number of roles. They were creating multiple roles per each portal, so the roles table had thousands of entries. In 1 location in code the entire table was read back and an attempt to cache the results happened, which caused problems. This code was refactored as part of the 4.5 efforts, so this scenario would now be possible (though IMO still not advisable - note: as you can now grant permissions at the user level, that requirement would no longer be necessary).

As for numbers of page, to use dotnetnuke.com as an example, AFAIK we have over 150,000 pages indexed by google, and I know of much larger sites that this. Realistically  your main limitation is really your amount of hardware, and as DotNetNuke supports webfarm setups that's really a finanical limitation more than anything else.

Cathal