Hi Greg,
There is not a lot of difference between running 1 site and 55, it depends on the aggregate traffic and how much server resources are available. You could have a 1 site installation that has more traffic than a 55 site installation. But of course if you expect very high traffic on all 55 sites you need a beefy server.
However it gets more complicated if the sites are all different domains and you want to use SSL, then you need a different ip address for each site that will use SSL because you can only bind 1 SSL cert to a particular IP address on the standard port 443.
In this case what you need is a separate IIS web site for each site that will require SSL, but each site can point to the same physical installation folder. If they won't be using SSL then all sites can run from a single IIS web site.
To be honest though I don't want t sound like I recommend hosting 55 sites in one installation, people do it, but I don't typically do that myself unless the sites are all for one customer and I don't have any running that many. Maybe others in the community can comment on that if they are running lots of sites in their installations. I know of a few people who have told me they run lots of sites.
If you have any obligation to make the data available to the customer, like if they want a db backup, there is not an easy way to extract the particular customer data if it is mingled in the same db with other customers.
There are no backup tools in mojoPortal, you backup the web site with file backup tools and you backup the database with tools specific to the db platform.
There are pros and cons about upgrading a multi site installation, the pros are less effort if you only have to upgrade one, but there is always potential for a little down time or errors that users may experience while you are upgrading the site and this could be a worse issue if it affects lots of sites or something goes wrong. If you plan to run a large installation then you should have a testing environments where you run a copy of the installation with a backup from the production db and test upgrading there and make sure everything still is working well before upgrading the production site.
Hope it helps,
Joe