‘Sambyaku’ (300) or ‘Sanbyaku’?

In writing the number ‘300’ in Japanese, I see ‘sambyaku’ in romaji used as the word for the aforementioned number in my textbook. However, I’m confused as to how I’m meant to write it (in consideration of both kana & kanji)— where would this ‘m’ letter come from? Wouldn’t ‘n’ be the only letter that could be used? As in, 「さんびゃく」 or 「三百」

Hopefully this is not something obvious where I’m missing the clear point, but I would still appreciate an explanation as to this confusion.