How many Federalist representatives are there whose first names are Benjamin?
Federalist refers to party = 'Federalist'; representatives refers to type = 'rep';
SELECT COUNT(T.bioguide_id) FROM ( SELECT T1.bioguide_id FROM historical AS T1 INNER JOIN `historical-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.first_name = 'Benjamin' AND T2.party = 'Federalist' AND T2.type = 'rep' GROUP BY T1.bioguide_id ) AS T