The minimum number of trading days needed for the challenges at FundedNext depends on the specific challenges.

However, it's important to note that there is no minimum trading day requirement in the Evaluation, Stellar 1-step, and Stellar 2-step FundedNext accounts. 
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On the other hand, for the Express FundedNext account, there is indeed a minimum trading days requirement of 10 trading days. Traders are expected to actively trade for a minimum of 10 trading days in the Express FundedNext account.

Please refer to the image below for details of the minimum trading days: