Sample size estimation

I have a text classification model trained on a highly imbalanced data with ~100 classes and got a decent over all F1 score. Now, I would like to validate how the model is performing on the new data set.

I don’t have any labeled data for the new data set. I want to find out the least amount of labeled data needed to validate model. The new data set may or may not have all the ~100 classes

Thanks!