There are several sample size calculators freely available online.

For example: https://clincalc.com/stats/samplesize.aspx

It all depends on the incidence of aphasia you expect, the error rate you find acceptable, and the power you want for your study.

Hope this helps you with your question.

Thank you very much Sir

Muito boa a sua colocação, tenho dúvidas semelhantes as suas.

Parabéns.

I think you can refer a guideline called COSMIN in which you can find how large a ssampl size should be for a validation study.

Thank you very much Sir! Zhongfei Bai