There is no setting in SharePoint where you would define which user (or how many) is internal (using CAL) or which one is external (without CAL). You pay for the internal user CALs, and externals come "for free".
So, you could put up a internet site with SharePoint and only purchase CALs for internal content producers. Usually companies do also have SharePoint intranet/other services on SharePoint, so they'd have CALs anyway.
You do also need Windows Server + SQL Server licenses, and Windows Server External Connector license. Some examples here.
In SharePoint you would differentiate internal and external users by using Zones. users coming through http://ext.company.com would be external users and belong to "Extranet" zone, while internals would come through http://int.company.com and belong to "Intranet" Zone.
Disclaimer: You need to verify these from a license reseller, your scenario might have special conditions.