What decides how to read a Roman number in titles?

Solution 1:

I don't think there's a real, well-defined pattern.

However, from observation it seems:

• A number, roman or not, after its subject is usually a cardinal number (e.g. "Chapter five", "World War Two").
• A number, roman or not, before its subject is usually an ordinal number (e.g. "Second World War", "25th Olympic Games").
• Regnal numbers, roman or not, are always ordinal numbers (e.g. "George the Fifth", "John Paul the Second", "Robert of the House Baratheon, the First of His Name" :-) ).
• Dates follow their own pattern because "June 2" is actually not "the second month June" but just "the second day of the month June" and as such the number is not related to the number of Junes at all.

This is not a grammatical rule, but it should be good enough as a guide on how to pronounce these titles.

Solution 2:

There is is no such general rule to govern how Roman numerals are read. It depends on widely accepted practices.

Observe the use in each of the following,

• Henry VII -- "Henry the Seventh"
• Star Wars: Episode II -- "Star Wars: Episode two"
• Saturn V -- "Saturn five"