They could be used to neutralize the positively charges or to enhance the ionic strenght. In the first case, it depend if your system molecules are positively charged or not. I don't know why you want to put anions on your system. If it is the second, as Rajnikant said, you need to add the same number of cations, it could be whatever you want, Na+, K+, Ca2+, etc.
You could work with a mix of both reasons, of course!