In order to balance this equation we need to have the same amount of elements on both sides of the reaction:
Cl2 (g) + 2 Kbr (aq) -> 2 KCl (aq) + Br2 (l)
Now we have the reaction perfectly balanced, as we can see the same amount of elements on the reactants side and products side.
The type of reaction can be found by looking for what is occurring directly to the molecules, for example, we see that the Chlorine gas is taking the bromine place and forming an ion with potassium, and that is basically all that happens in the reaction, and this will be an example of a single replacement reaction
One better example to identify a single replacement reaction is:
A + BC -> BA + C