Gerbils are social animals and need the constant company of their own kind. The only exception to this is the Fat-tailed gerbil, as females of this species can be kept alone, and will often fight with other females.
Otherwise, it is always advisable to get a pair of gerbils, either two males or two females. It's true that they can live in larger groups, but unless you have a very big space to keep them in, they're going to be a bit crowded. In general, it's best to keep between two and four gerbils. Also, large groups can become unstable as individuals jostle for a place in the hierarchy, and aggression can sometimes break out - especially if there is not enough space. Most fights are due to dominance, and problems are more likely if you keep females in larger groups.
How many gerbils can you fit in a teapot - or any other gerbil space, for that matter?
You should try to buy gerbils that are already in pairs - siblings, usually - as it can be difficult to introduce two gerbils when they are adults (i.e. around 16 weeks old). Although gerbils are social animals, they are still territorial and will fight 'strange' gerbils who enter their space.
At eight weeks old it is still possible to introduce gerbils to each other. They are still babies and will mix happily with one another. Once they are adults, it may be possible to pair a male with a gerbil pup, but this requires care and constant monitoring for the first 24 hours. For this reason it is best to get your pair of gerbils from the same place, at the same time.