g=1;k=0.001;f=0.1;p=0.5;
sol=NDSolve[{n'[t]==k(p/k-f/g-n[t])g n[t]/(f+g n[t]),n[0]==1},n,{t,0,100}]

Plot[n[t]/.sol,{t,0,100},PlotRange->{{0,100},{0,100}},AxesLabel->{Style[t,Large,Bold],Style[n,Large,Bold]},LabelStyle->Directive[Large,Bold],PlotStyle->Thick]

0.001*0.1