First of all, the convention is to list males first, females second. So 1 male, 2 females would be 1.2. Just trying to avoid confusion.

It depends on whether you intend to leave the entire colony together constantly, or rotate the males in & out and/or separate the females into birthing tubs.

I believe some people do as much as 1.4 for breeding purposes, but if you imagine 4 litters of babies growing up in there, you can easily see that would be WAY too crowded. I think 1.2 is more typical for those that leave the colonies together the whole time.