The ammount of mice you should have completely depends on the cage.
Males have been reported to fight sometimes if kept together, but if introduced at a VERY young age, this isn't a problem. I have my two males housed together- no problems there.
Even with good care, a typical mouse only lives 1.5-2 years.. though there can be exceptions.
I find my males to be a bit sweeter, though females have their good points, also... and can be just as sweet. I'd say if you want multiple mice, go with females this time... just to make sure you won't have to separate them in the future.
A 10g would be good for two female mice. =)


A little side info:
Mice aren't like hamsters. They get obese rather easily. And obesity leads to health problems. They're prone to the typical diseases: URIs & Tumors. I haven't had any URI problems with mine, but one of my girlies has a tumor.
Food-wise.. I'd say don't get a seed mix like what hamsters normally have. For mine, I feed them a small ammount of my ratfood mix(homemade) and plenty of mineral blocks. As to assure they get all the nutrition they need.
They really love shredding cardboard, so cardboard tubes(like from paper towl rolls and TP rolls) are a must!

And...
I have my females in a nice-sized wire cage(super small bar spacing). They really love it. My males are in a 10g.