Since it's not specified whether Magnus can have stickers left over:

This is the solution if you don't have to hand out *every sticker.*

a + b <= 12.

Create a dummy variable, k, for all leftovers

a + b + k = 12.

This is great because we see k has to be at least 0, like all the others.

Apply stars and bars formula:

\({12+3-1\choose 3-1}=91\).

There are 91 ways.

This is the solution id you don't have to hand out every sticker.

Give one friend any number of stickers 0 -12.

The other gets all the rest.

There are 13 ways.