They would be pleased to see the number of tourists decrease.
"They would be very pleased to see the number of tourists decrease."
「観光客が減ると彼らはとても喜ぶでしょう」
* be pleased: 喜ぶ
* to see: 見る、知る
* number of: 〜〜の数
* tourists: 観光客
* decrease: 減る、減少する
ご参考になれば幸いです。