@WBYeats,
WBYeats wrote:
Is the S optional and I can use it as I like?:
-For those students, there are only several of them whose name(s) I am not sure (about/of?).
The 'only several' is misplaced; if you wish to emphasise that you are sure of most of the names you could use 'only a few'.
'of them' is optional and often omitted.
The plural 's' is necessary and not optional if there are more items than one.
For those students, there is only one whose name I am not sure of.
For those students, there are only a few whose names I am not sure of.