I think I might have made a mistake regarding the number of days I planned to stay.
I think I booked the wrong number of days for my stay with you.
この場合、次のような言い方ができますよ。
ーI think I might have made a mistake regarding the number of days I planned to stay.
「滞在予定の日数を間違えたかもしれません」
the number of days I planned to stay で「滞在する予定の日数」
ーI think I booked the wrong number of days for my stay with you.
「ホテルで滞在する日数を間違えて予約したと思います」
to book「予約する」
the number of days for my stay で「滞在日数」
ご参考まで！
カナダ 
