Tens of thousands.
At ISC schools alone (which represents the majority of, but not all independent schools)
”103, 000 pupils at our schools have special educational needs.
55, 000 pupils are identified as having Specific Learning Difficulty (SpLD) and 7,000 pupils have an Education, Health and Care (EHC) Plans.”
There are many more who have educational needs or mental health issues without having a diagnosed SEN.