What's the maximum number of Global Address Lists (GALs) that we can create in our Exchange 2000 Server organization?

You can create a maximum of 800 address lists for each Active Directory (AD) forest. This limitation is directly related to AD's limit on the number of values in a multivalued attribute. However, if your forest is in Windows Server 2003 functional mode, you can create a maximum of 1300 GALs, although I don't recommend creating that many GALs—in fact, I've never seen an organization (except for hosting service providers) with more than a few dozen GALs.