How do I limit the number of products made available to feeds?

Products are made available to feeds in groups. When you create a new group, you can either include the whole products portfolio, or filter the group by collection(s).

You can define as many groups as you want to.

In each of the groups, you then define feeds, which target one of the Google Shopping markets. In the feeds you define important parameters that help Google better targeting your customer, e.g. product category, age group,  gender and so on.