Start with 10^6 cells. Say 0.1% (1 in 1000) are already mutants at that site. Then add something that kills 100% the non-mutants and you will be left with 10^3 mutants without any gene editing. Say it kills 50% of the non-mutants and renders the rest quiescent due to DNA damage (not dividing), then you are left with 10^3 mutants and 5 x 10^5 non-mutants at time t0. After eg 7 divisions you will have 10^3 x 2^7 = 1.28 x 10^5 mutants, corresponding to 25% of the total.

It depends on the initial number of cells, initial proportion of mutants, division rates, and toxicity. I have also noted that the initial number of cells is usually reported without any uncertainty, which makes me think those numbers may be rather unreliable.