Shrimps need a male and female to produce young. But you can get away with buying just one shrimp if it is a female which is carrying eggs under her abdomen - a berried female.
Most shrimp eggs hatch into miniature adults. But amano shrimp eggs hatch into a planktonic larval stage which needs to be in salt water to develop. If you buy amano shrimps, you will never get more shrimps, but with other species, they will increase in number provided you have at least one male and one female.