I gave mine a smallish piece everyday. How you will know if you are giving her too much is if she gets diarrhea. And then you would just not give her some for a few days until her stool goes back to normal.
It is reccommended to get female compainions, as females live longer happier lives when is groups of at least two. I have heard however that large groups sometimes become aggressive with eachother. I had a group of 4 once that suddenly ganged up on one of them and chewed her ears off.
Also, some females are just plain mean. You would obviously have to make sure their personalities match.

She is most likely to get along with a young mouse. And of course you would have to put the new one in QT for about 30 days to make sure it is not ill.
I would say to do some research on intros with mice, as there are a few ways to go about it. You could put their cages next to eachother, then maybe give each one bits of the others used bedding to get used to smell, and then let them vist eachother in a place neither of them has ever been. Then if all goes well put them together and keep close eye on them (after cleaning out the cage and putting all new or cleaned stuff in of course). And to lessen the risk of fighting, when they are in the same cage put two food dishes and two water bottles and maybe two beds so they do not suddenly feel cramped together.