What are the SQL Server Express and MySQL limits?

Both have no database limit.

SQL Server Express can even have databases up to 10GB each. The full version does not have this limit. MySQL only has a practical limit of the operating system that can be 2TB in most cases. But this may be the size limit of each table depending on how you are using it. And the tables can be partitioned.

SQL Server (including Express) can create up to 999 traditional secondary indexes per table, and MySQL can create 64 sub-indexes per table. But never create so many. The size of the index file depends on the size of the database.

Remember that SQL Server Express has business imposition limits. The main one is the memory size that can be used for buffers and other operations that is only 1GB.

Most of the limits of almost all major database software are sufficient for almost every possible application. Today the limits are more hardware or practical.