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This depends on the context. If the ordinal is quite low (first to fifth, say) and you aren’t using many of them (e.g. “we’re the second-largest company in Saudi Arabia, and the fourth-largest in the world”) then it’s best to spell it out. If there are a lot of ordinals and they vary a lot, then it’s best to use numbers.
Basically, spell it out unless it seems too high a number or there are too many of them.