There's no single definitive answer to how many secondary schools are in London. The number fluctuates slightly as schools open and close, and different organizations use different criteria for what constitutes a "secondary school" (e.g., including or excluding academies, sixth forms, etc.).
To get an accurate figure, you would need to consult the most up-to-date data from the Department for Education (DfE) in the UK, and even then, the definition used will affect the final count. You'll likely find a range rather than a precise number.