Your school type affects the number of INSET days
You can set up to 5 compulsory INSET days during a working year - this is stipulated in the School Teachers' Pay and Conditions Document (STPCD) (section 51.2).
See the second section of this article for more details on this.
You can decide how many INSET days to set during the year, but bear in mind that you won’t be able to make it compulsory for any teachers employed under the STPCD to attend any additional days on top of their specified working days.
The law around the number of teaching