。
She stayed there for a few days.
Literal
She [topic-は] there [at-に] several-days stayed.
数日 ('several days') uses 数 ('number / a few') as a productive prefix on counters: 数人 ('several people'), 数回 ('several times'), 数年 ('several years'). It typically means somewhere in the 2–5 range — vaguer than 'a couple' but smaller than 'many'. 滞在する is the Sino-Japanese verb for staying somewhere temporarily — at a hotel, a city, a country — and pairs naturally with travel contexts. The に marks the place of stay (location of existence/state), not the destination of motion.