Basically any server configured as a DC can be a global catalog.
There is no need to have more than 2 in a single site domain, its not best practise as this can cause big replication traffic, but if you have multi sites,multi domains, you can dump a DC in each, set it as a global catalog and this is all good especially if your site links are pretty slow.
You can also play about with Universal group caching if on 2003.
Good link on the topic below
www.microsoft.com/resources/documentatio...cs/en-us/gc_when.asp
