Are 0, 1, 2, 3, 4....... also considered Romaji? If they are not written using kanji?
No. ローマ字 (Roman script) is actually an alternative name for ラテン文字 (Latin script), which only refers to A, B, ..., Z. In Japanese, ローマ字 also means transcribing Japanese words using Latin alphabet, but that's an extension of the original meaning.