There are pre-defined limits depending on whether you are using a ‘Domain-share’ or a local one. Domain shares are less limited (50,000 folders i think, no file limit).
If you exceed this you can create more domain shares to accommodate. Replication won’t be an issue but obviously the more you have to replicate the longer it will take.
The limits are also different between a 2003 and a 2008 environment (lower in 2003).
This would be a good place to start