The event was a big success. There were 〇 people waiting for the reservation cancelation.
The event was a big success. As many as 〇 people were on the waiting list.
It was such a successful event. There were 〇 people on the waiting list.
キャンセル待ちは
on the waiting list （待ちリストに名前を載せる）
waiting for the reservation cancelation （予約キャンセルを待っている）
と回答しました。
The event was a big success. は
イベントは大成功でした。という意味です。
It was such a successful event. も同じです。
It was such a happening event, there were almost 30 people waiting to get in.
この場合、次のような言い方ができますよ。
ーIt was such a happening event, there were almost 30 people waiting to get in.
「すごく大盛況のイベントで、30人ほどが入るのを待っていた」
a happening event を使って「大盛況のイベント」を言えると思います。
ご参考まで！
日本 
カナダ 
