How to apply supermesh to this?
I think maybe you have only ever seen circuits where the current source is sitting in a leg shared between two loops. In that case, you do need to use a supermesh because a loop equation containing unknowns cannot include the current source since the voltage drop across the current source is unknown. Since it is an extra unknown, it is difficult to solve if you make it appear in the loop equations. 

Therefore, the approach is to assign the current value of whatever current loop you decide the current source will be part of (it could even be a super loop if you wanted) to a known value (the current source's value). This prevents the voltage drop of the current source from appearing in the loop equation.

But since the current source is sitting in a shared leg, another loop equation must account for it as well (and account for components that have been left out somehow). You obviously don't want another loop equation to contain current source's unknown voltage drop, but you also can't just set a assign a second current loop to be the current source's value. So what do you do?

Well, what you do instead is you make a super loop that encompasses, but bypasses (i.e. does not run through) the leg that the current source is in so you can account for every component in the circuit, but not have the current source's unknown voltage drop sitting inside any loop equations. This is why you need a super mesh.

But it is obviously not universal, as your example circuit demonstrates. If the current source is not in a shared leg, you don't need a super mesh. In this question, the current source is not in a shared leg. Therefore, after you set the current circulating inside the current source's loop to be a known value, there is no need for the current source's unknown voltage drop to appear in any other loop equations so you can go ahead and assign the rest of the loops however you want. They can be individual loops or super loops or whatever.