NGS Style Manual‎ > ‎- S -‎ > ‎

second, third, fourth, etc.

Spell out ordinals through tenth. Use figures with ordinals larger than tenth as follows:
            21st, 32nd, 43rd, 54th

Do not hyphenate ordinals when part of a comparative or superlative.

second largest city
tenth longest river
101st time
second-generation Italian
third-place winner
third-ranked convention city