This blog post (and the one that it references) will tell you pretty much everything that you need to know about Windows Azure Compute Instance and Storage limits.
These default limits are:
20 Hosted Service Projects 5 Storage Accounts 5 roles per Hosted Service 20 CPU cores across all of your Hosted Service Projects
Remember though that these limits are just the default and higher limits can be negotiated. If you have an application that needs more it can be arranged.