In active-model-serializers, how to limit the associated objects returned from a has_many association?

So I have

    render json: Post.all       

This returns all my Posts and in my Post serializer I have

    class PostSerializer < ActiveModel::Serializer
      has_many :comments
    end        

I want the number of comments returned in the JSON to be limited to 5 and have a variable which tells if more comments are there. Is this possible?

Edit: I think I'll manage the more part with a new call. But can't figure out how to limit the comments in the serializer