数 + [measure word] / 数十 / 数百【shù / shùshí / shùbǎi】

数, read shù here, is the written word for "several". It takes the place of 几 in a statement and goes straight in front of a measure word, or of a time word like 年 or 日 that needs none.

With round numbers

In front of 十, 百, 千 or 万, 数 gives a rough figure: 数十 is "dozens", 数百 "several hundred", 数千 "several thousand". This is where you meet it most, in news reports and figures.

In speech the same figure is 几十, 几百, 几千. 数 belongs to writing and formal announcements; in a chat it sounds stiff.

Where it differs from 几

几 is also "how many?". 数 is only ever "several" and never makes a question. And since the two do the same job, they never stack.

• × 你在北京住了数年？ → 你在北京住了几年？
• × 数几天 → 数天 / 几天

As a verb, 数 is shǔ, "to count": 数钱, 数一数. Before a measure word or a number it is always shù.