Especially during festive seasons, the temptation for kids to want more and more stuff is always there, leading to overflowing drawers, toys all over the floor and under the bed, and just stuff lying all around the house. Have a rational discussion with them and get them to agree on what would be a reasonable limit on the number of toys - for example, three boxes full of toys.
When that limit is reached, they either have to give away other toys to make space for new ones, or they have to forgo new toys. This also teaches your child from a young age not to be a mindless consumer, but to think about whether they really need or have space for new stuff.