help calculating needed sample size

hi im conducting a study on a certain ratio in a blood exam (lets call it X) .

I aim to say that there is a certain sensitivity (let say>80%) for people to be sick if X>10. (my actual intent is to check the sensitivity for different values of X)

its a retrospective cohort and the estimated incidence of sick people in the whole group is 3.5%

how or which tool should i use to calculate the sample size? i have tried to find an answer online (including this good chinese site: Size) but so far no success

thanks so much for your help!