@Teng Wang Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
I understand that you want to add more than 400 URLs to a custom Bing search.
Unfortunately this is a hard limit. This article talks about the limit being 400 urls that can be used as ranking adjustments for both active and blocked. This is the limit for a custom search instance, and it cannot be increased due to the design of the service.
Hope this answers.
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