how many Number of threads can be Created in STM32F207. I have created 6 threads successfully but when i am trying to created 7th thread its not working. On which factor thread number is defined how to increase it?

From the API documentation for the xTaskCreate() return value:

"If the task was created successfully then pdPASS is returned. Otherwise errCOULDNOTALLOCATEREQUIREDMEMORY is returned."

So the task was not created because you ran out of heap memory?

Then read http://www.freertos.org/a00111.html which says how to allocate more memory to FreeRTOS - see the configTOTALHEAPSIZE parameter.

Googling configTOTALHEAPSIZE then also provides a little more information and links: http://www.freertos.org/a00110.html#configTOTALHEAPSIZE

Hope that helps!