For Covid-19 situation (online Pre-sessional)
If Pre-sessional courses are online they will likely to be taken from the student's country of residence in which case a CAS (confirmation of acceptance for study from UKVI) will not be necessary for the Pre-sessional courses, unless residency is sought in UK during the time of the Pre-sessional. Only one CAS will be necessary, for the main degree course.
However if courses go live on campus in 2021 then for Undergraduate and Postgraduate programmes there is the option of an integrated Pre-sessional for which one CAS is needed. For Doctoral School the Pre-sessional courses are not integrated and will require a further CAS.