Microsoft also released a new set of SQL Server 2012 Express editions. SQL Server Express is a free edition of SQL Server, and it’s used as the internal database for hundreds of third-party products.
Although it's free, however, it also has several limitations. Compute-wise, the new SQL Server 2012 Express editions are limited to the lesser of one socket or four cores. The editions are also limited in size. The maximum memory for the database engine is limited to 1GB and the maximum relational database size is 10GB.
Although SQL Server Express is limited in scalability, a common misconception is that it’s limited to a single database or 10 connections. Neither of these is true. SQL Server Express can support multiple databases where each database can be up to 10GB. The myth about the number of connections is probably a holdover from the early days of MDSE (Microsoft Database Engine—the first version of the desktop SQL Server Engine that was released in the SQL Server 2000 timeframe.)
SQL Server 2012 Express has no limits on incoming connections or workload other than the CPU and memory limitations. And it comes in both 32-bit and 64-bit versions. You can download all of the SQL Server 2012 Express editions from Microsoft’s website.
Reference : http://www.microsoft.com/betaexperience/pd/SQLEXPCTAV2/enus/default.aspx