I haven't had gerbils for ages and have been sorely tempted though. They are great for older children (even better for adults in my opinion!) as they are cheeky, brave and do lots of fun things, and of course are gorgeous to look at. They also have the advantage of not being specifically nocturnal so you can hang out with them more.
It's been a while since I've had them so probably good to google for a reliable website with lots of info, but "back in the day", the rule was always get a pair if female and 2 or 3 if male (opposite of the rule for guineas.) I've had both combinations happily.
The last gerbil I had was a rescue, and a solo male gerb who lived in a tank filled with layers of peat and straw (which only had to be changed every 3 months.) He was fairly feral (having had the run of a house for about 6 months before being "captured") and at least at the time, the advice was against introducing new companions to adult gerbils. But, he seemed quite content tunnelling around, growing and eating grass that sprouted from discarded bits of food and staring at the guinea pigs "next door."
Previously, I had gerbils in large plastic enclosures or cages with bars and on sawdust. I know now the cages were not a good idea - some tended to chew until their noses were sore and also, the digging in corners I later discovered was a sign of boredom/frustration.
I'm not sure what the ideal solution is for housing. I think the tank idea is as close as you can get to replicating their natural environment, but it doesn't make it easy for catching/handling them. In an "open" plastic enclosure or cage, you can catch and handle them easily, but the rest of the time they are likely to be frustrated in a very artificial environment.
Definitely have a look on google. Good luck!