This is the 346th most frequent Japanese word.
枚
Counter for flat objects.
In this sentence, the counter '枚' (mai) is used to count one flat item, which is a plate in this case.
テーブルにお皿が一枚あります。
There is one plate on the table.
Here, the word '枚' is used as a counter for flat objects, specifically photographs in this case.
彼は写真を何枚持っていますか？
How many photos does he have?
The counter '枚' is applied here to quantify two flat textbook items arranged on the table.