Three distinct senses sharing the same word: cardinal numbers (一, 二, 三...) vs. ordinal 序数 (第一, 第二...); the math 'base' of a number system; and a baseline / reference figure (人口基数, 工资基数). Context disambiguates.
grammar
Watch the contrast pair: 基数 (cardinal: one, two, three) vs. 序数 (ordinal: first, second, third). A common test point in Chinese math/grammar.
