When you create a site collection you can specify one or two site collection administrators on the Create page.
When you enter the second site collection administrator's information, no third entry box is opened, two is all you get.
But what if you want to have more than two?
One possibility is to specify AD groups in those two entry fields instead of single user names.
But a better method is to open the site collection (after it's been created!) in your browser and via the User Interface do the following.
- navigate to the top-level site in the site collection
- click "Site Actions" >> "Modify all site settings"
- under "Users and Permissions" click "Site Collection Administrators"
- add new administrators there
Thanks to Darrel for posting this and giving me permission to use it (even if he did get it from his MS System Architecture Rep!).