I adopted one gerbil and then I asked the shop staff if I can buy another gerbil for him and he said no cause they might fight!
But can I actually buy one?

The FIRST thing to do is get several books from the library to learn about how to care for a gerbil BEFORE you buy one. Two in the same space will fight. Yes, one is perfectly happy by itself. We gave our gerbil a wheel to run on and gerbil toys to play with.