There are a number of ways that could cause cockroaches to appear in your home. They can be transported in through bags or boxes. They could come in from under the door or through plumbing. It does not take long for them to multiply.
1 Additional Answer
An untidy home or area will cause a cockroach outbreak. Cockroaches are scavengers, so crumbs and food particles left out for them to feast on will cause them to thrive. And where there is one, there are hundreds so beware.