There are two options. First, you can mash some of the potatoes and mix those into the chowder to thicken. Another alternative is to use a thickener. If you use cornstarch, it's best to whisk it into the cream before adding it to the pot. Pour the cream mixture into the chowder, whisking to make sure the cornstarch doesn't clump, then bring to a boil, stirring until it thickens. This recipe uses flour to thicken the chowder so no need for either intervention.