数万

数 here means 'several' (an indefinite small number, typically 3-9) and combines with 万 ('ten thousand') to give 'several tens of thousands' = roughly 30,000-90,000. Same pattern: 数十 (a few dozen), 数百 (a few hundred), 数千 (a few thousand), 数亿 (several hundred million). All written-register; spoken Chinese prefers 几万.

mistakes

Don't read 数 as shǔ (3rd tone, 'to count') here - in this set-phrase context it's shù (4th tone, 'a number/several'). The reading shifts with the meaning.