Define a calculation field in your table as:
If ( IsEmpty ( FilterValues ( ValueListItems ( Get ( FileName ) ; "YourValueListNameinQuotesHere" ) ; YourCheckboxFieldHere ) ) ; "Other" )
Select Text as the result type.
There are then several different ways that you can count all records where this calculation field returns "Other". Which method you can use, depends on the version of Filemaker and the context in which you want to get that count.