Probably depends on the age and individual conditions the stubbies been in, I’ve never reactivated the yeast myself but I know of people who say that 3 stubbies worth is plenty, coopers recommend 6 stubbies but they’re using a PET bottle rather than a stir plate which might affect results.
Might have to give this a go on the next brew.
Make second step 1.04 to give it a bit more to munch on. I reckon that should get you enough for a single batch yep. Not sure exactly about cell count I just go on experience!