In order to keep your MOSS 2007 solution fast and scalable you should take the following guidelines at heart:
If you look at the "Guidelines for acceptable performance" the first thing you should be aware of is that Microsoft makes a distinction between "web sites" and "sub sites". Where web sites are actually the first sub sites under a site collection, and sub sites are the sites under the web sites.
The total number of web sites and sub sites together in one site collection should be 250.000. So if you've got less web sites, you can have more sub sites. Keep in mind never to use more than 2000 sub sites per web site.
Another number that stands out and causes some confusion is the number of 2000 security principales per web site. Security principales are users and SharePoint groups. I asume that the maximum number of 2000 is per site collection since all groups are cross site groups in MOSS.
The last thing you need to be aware of is that a content database can store several site sollections, but that one site collection will always be stored within one content database. In order to be able to do a database backup and restore in a reasonable amount of time you should keep the size of your database limited. I always aim for about 50Gb maximum. This means you have to think about how much storage your site collections will use in order to determine the maximum number of site collections per content database.
You can set the maximum number of site collections per content database in the Central Administration. Go to Application Management and click Content Databases here you can set the "Maximum number of sites that can be created in this database".