Binomial distribution example

In this case we don’t know the total colleagues in his office, so if we are interacting with 50 colleagues won’t the probability of the being infected (the given probability = 0.1) change depending on which sample of 50 we have met?

and what if the case that there are 250 colleagues (10% is 25) so in this case we cant assume n to be 50 , as its not possible to meet 50 infected, so will the formula still work on this case?