Introduction to keeping more than one hamster
When you first get a hamster, you often just start with one and might want to add more. This might be possible, depending on the breed of hamster you have. It is best to get littermates and raise them together. This will not completely eliminate fights and territorial squabbles, but it should help prevent serious injury or death.
Syrian hamsters (also known as golden or teddy bear hamsters) are extremely territorial and should be kept in solitary housing conditions. They are not social at all and will sometimes fight to the death. At the very least, one will become very dominant and will not allow the other to eat and roam freely. If you are wondering how pet stores house so many Syrians together, it is because they are most likely litter mates and the population is constantly being sold and replenished. (Even so, fights do occur, but they usually happen at night when no one is around.) Females are especially vicious towards males, and mating pairs should only be allowed to be in the same cage for a short time while the female is in heat. Once they have mated, remove the male and return him to his cage. Syrians become territorial at around 5 or 6 weeks of age and begin to fight. They should be separated at that time.
Dwarf hamsters are more social than the Syrians, but do not place hamsters of different species together because they will most likely fight to the death. Dwarf hamsters are fine whether alone or in small groups. If you have a mating pair, be sure to remove the male when they have a litter. He will not harm the pups and might even help the female with them, but dwarf hamsters can become pregnant very soon after having a litter, so it is best to keep them apart so the female does not have several litters in quick succession. It is more practical to have a pair of same-sex hamsters than to have a male-female pair unless you plan to breed them.
Dwarf hamsters tend to be more active if they are not housed alone, so having a same-sex littermate for your dwarf hamster is not a bad idea. Solitary dwarf hamsters tend to be less playful and active and not as interesting as a pet. Even dwarf hamsters will fight, but this is not usually serious and is only done in order to establish a hierarchy. You will hear a lot of squeaking and rolling around as they establish dominance and then it should become a bit quieter. If you have a solitary dwarf hamster, it will be more difficult to introduce a new hamster later. If you have a pair and one hamster dies, you might be able to introduce a new hamster at that time.
Whether you choose to have a solitary Syrian, many Syrians in separate housing, or lots of dwarf hamsters in the same housing area, your choice of how many hamsters to have is only limited by your space, resources and patience.