What is the available SQL Server Express database size?
I haven't used SQL Express personally but from my understanding of how Microsoft's licensing works you'll be able to grow to the 10 GB limit, but then if the file tries to grow any further the operation will fail.
The limit is the allocated size of the MDF file. So technically you have about 2 GB left. Once your MDF is 10 GB, even if not all of that space is used, you're at the limit per the licensing agreement. You should still be able to use the server and fill the unused allocated space up though (without incurring any further file growths) and still be compliant.