Chinese makes ordinal numbers by prefixing 第 to the cardinal: 十九 (19) → 第十九 (19th). The ordinal then takes a measure word like 名 ("place/rank"), 届 ("session/edition"), 课 ("lesson"), 章 ("chapter").
culture
In post-1949 Chinese politics, 第十九 implicitly refers to the 第十九届中央委员会 / 第十九次全国代表大会 — the 19th National Congress of the CCP, held in 2017. So in news and political contexts, 第十九 is often a shorthand reference to that era.
