Postgres：多対多のリレーションテーブルで最小行数と最大行数を取得します

ベンダーの金額をウィンドウ関数の最小値および最大値と比較し、外部クエリレベルで条件付きカウントを実行します。

SELECT vendor_id
     , count(min_bid OR NULL) AS min_bid_count
     , count(max_bid OR NULL) AS max_bid_count
FROM  (
   SELECT vendor_id
        , amount = min(amount) OVER w AS min_bid
        , amount = max(amount) OVER w AS max_bid
   FROM   rfq_vendor_mapping
   WINDOW w AS (PARTITION BY rfq_id)
   ) sub
GROUP  BY 1
ORDER  BY 1;

SQLフィドル。