It’s difficult to give you a simple yes or no with the information provided, but I’ll give you some information that should help.  SQL Express is limited by a number of factors: 1 CPU, 1 GB Ram, 4 GB database.  Also, beginning with SQL Express 2008 R2 the database size was increased to 10GB, so that’s considerably more space depending on what version you are referring to.  

 
The number of documents doesn’t directly affect the size of the SQL database since the files themselves are stored on file servers.  Metadata, security, and other repository information is stored in the database, the size of all this information is dependent upon how it’s used, which is why it’s so hard to give you a strict yes or no answer.

Beyond size limitations, our typical recommendation is that SQL Express not be used for a system with more than 10-15 users.  If you have a large amount of users performing taxing commands like searching and importing then SQL Express would most likely be insufficient.  The nice thing about SQL Express is that you can always start out with it and then upgrade to full SQL if it isn’t capable of handling the system load.