Ten.
Here's the list: https://k1.spdns.de/Vintage/Sinclair/82/Peripherals/IF2%20Cartridge%20Interfaces/Sinclair%20Interface%202/
And another: https://spectrumcomputing.co.uk/entry/1000193/Hardware/ZX_Interface_2
Broken down by date: https://k1.spdns.de/Vintage/Sinclair/Software/ZX%20Spectrum%20Software/Interface%202%20Cartridges/
And another: http://rk.nvg.ntnu.no/sinclair/computers/peripherals/interface2.htm
Here's some info about 3rd party diagnostic ones and so on: http://www.fruitcake.plus.com/Sinclair/Interface2/Cartridges/Interface2_RC_Cartridges.htm