がたくさんあった

there was/were a lot of

→ が
phrase Rare #23,797
there was/were a lot of Used to say that something existed in large quantity or that someone had many things to do or deal with.
JA: 机の上に本がたくさんあった。
EN: There were a lot of books on the desk.

私が行った日には空席がたくさんあった。

There were plenty of empty seats on the day I went.

彼らから自発的な申し出がたくさんあった。

There were plenty of unsolicited offers from them.

家の中には食べ物がたくさんあった。

There was plenty of food in the house.

見ておもしろいものがたくさんあった。

There were lots of interesting things to see.