You can have different virtual directories share the same port number but they must have a different “host header.” This means that you need to assign a different DNS entry/URL for each. I use a mechanism like firstportal.mydomain.com, secondport.mydomain.com, etc. Which isn’t too different from what you were doing before which was probably mydomain.com/firstportal, etc. The only downside to this new approach is that you have to coordinate it with the DNS. You can either have the machine run DNS or you can have the upstream DNS have both of these point to the same IP address. Either way, when the HTTP request gets to IIS, it will direct it to the correct web site.

I don’t know if there are any hard coded limitations on the number of portals, but there are certainly practical ones depending upon the numbers of users and power of the machine.

Yes, all document libraries are exposed automatically as what MS used to call web folders and now seems to refer to as network locations or network places. There is also WebDAV support.

Good luck with the migration.