None! Feed her ratsIf you have a bunch of mice in stock, I'd use up those, but it's much more economical to feed rats in the long run. At her age, she could easily take two.
The issue is that some balls won't take more than one item per feeding. I have one like that. I also have one that eats everything you throw at her. Try offering her seconds after she's gotten down the first mouse. If she doesn't take two, she might be a one-item-per-feeding kind of gal.