It varies depending on whether you have online Catchup+ activated or not? You can view the total number of titles as follow though:
Press the blue DSTV button on the remote and scroll right to Help
Scroll down to General Information and then select Information Central
With the top line highlighted, press OK and then select VOD Status
There will be three tabs showing the number of CU, CU+ and BO titles currently available on your Explora