Only use a numeral if the number cannot be spelled in two words or less. For the roman numeral "one", use capital "I"; for the Arabic numeral "one" use "1". Never begin a sentence with a numeral, but rewrite the sentence.
25,500 voters hailed the passage of the bill. The bill was hailed by 25,500 voters.
Roman numerals are required for:
1. Main idea
a.Subidea
b. Subidea
1. Example subidea
2. Main idea
Vol. II of Wrinkler's Writing a Paper
Book I of ...
Roman numerals are not necessary for:
Chapter xii of D. Hendon
Page vii from Preface to Georg Katkov's
1917: The February Revolution. Russia