How to limit the number of documents in a mongoose fetch?

Question

MongoDB

Hello! I'm trying to make a database fetch script using mongoose. What I want to do is have a collection that contains posts that have different categories. I need to get posts from each category, and 5 posts each. limit() is not suitable, because the code will become much more complicated (you need to find it in the current category, wait for the answer, the next one - can it be simpler?)
I did this:

const Posts = mongoose.model('post');
    const Categories = mongoose.model('category');
    // create request obj
    const findPostsRequest = (categories) => {
        if (!categories.length) return {};
        let request = { $or: [] };
        categories.map(category => {
            request.$or.push({ 'categories.link': category.link });
        });
        console.log(request);
        return request;
    };
    // find all categories
    Categories.find()
        .then(categories => {
            // find 5 posts in every category from categories
            Posts.find(findPostsRequest(categories))
                .then(posts => {
                    res.status(201).json(posts);
                })
                .catch(err => {
                    res.send({
                        message: `Ой ошибка:  + ${err.message}`
                    });
                });
        })
        .catch(err => {
            res.status(400).json({
                message: `При получени записей произошла ошибка:  + ${err.message}`
            });
        });

That is, we get all categories, and then add category conditions in one query with an array OR. I need to shove $slice somewhere, but I still don't understand where. You need to get 5 posts from each category. How to do it?