Is there a bus from Xi'an to Zhongwei?

Do you mean the Zhongwei County at Ningxia? Well, I heard the bus between runs once every two days. Comparatively, to take a train can be more convenient. As I know, there are four trains running every day, the schedule, fare and duration can be searched at the website:
https://www.travelchinaguide.com/china-trains/
Hope help!