Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE CMI_cross_references ( source_system_code VARCHAR )

### Question ###
How many different source system code for the cmi cross references are there?

### Accurate SQL ###

SELECT COUNT(DISTINCT source_system_code) FROM CMI_cross_references