In FileMaker 12, ExecuteSQL can selectively count the related records.
Without ExecuteSQL, there are three approaches that can give you the needed count:
1) define a new relationship between the two tables. Use both the Style field and this calcualtion field: constAdopted as match fields in this relationship. constAdopted can be a calculation that just returns the text "adopted" in every record of Workhorse Styles.
With this approach both a count function in a calculation in a workhorse Styles field or a "count of" summary field defined in Workhorse Data can provide the needed count.
2) define a calculation field in Workhorse Data, cAdoptedFlag as: status = "adopted" with number as the result type.
Now either a Sum function (workhorse styles) or a "total of" summary field (workhorse data) that totals the cAdoptedFlag will count the related records with status = "adopted".
3) if you just need to display this count, you don't need to use it in a calculation or export it, you can use a relationship based on the style fields and put a "count of" summary field inside a one row filtered portal. The portal filter expression:: Workhorse Data::status = "adopted" would then be used to filter out all but adopted records from the count.
#2 is the best option for me, it is similar to what I was trying to do but I was missing the sum function part. Thanks!