How to return a value using callback

Question

I have a problem with my code. Any solution that I found are unclear to me.. I want to get a value by returning it by fethcing my database but obviously I don't handle my callback well.

Here is my code :

nbr_users = (key) => {
    db.User.find({
        shop: key
    }, (err, users) => {
        if (err) res.json(err)
        else
            return users.length //it's the value I want
    })
}

module.exports = (req, res) => {
    db.Account.find({}, (err, accounts) => {
        if (err) res.json(err)
        else {
            var accountMap = {}
            var i = 0;
            accounts.forEach((account) => {
                accountMap[i++] = {
                    users: nbr_users(account.key) //here I call my function
                }
            });
            res.json({user_list: accountMap})
        }
    })
}

EDIT: I don't know how to use async/await I try this, I have no idea about how to fix it

function getUsers(key) {
    return new Promise(function(resolve, reject) {
        db.User.find({
            shop: key
        }, (err, users) => {
            if (err) res.json(err)
            else
                resolve(users.length)
        })
    })
}

nbr_users = (key) => {
    return await getUsers(key)
}

module.exports = (req, res) => {
    db.Account.find({},
    (err, accounts) => {
        if (err) res.json(err)
        else {
            var accountMap = {}
            var i = 0;
            accounts.forEach((account) => {
                accountMap[i++] = {
                    users: nbr_users(account.key)
                }
              });
            res.json({user_list: accountMap})
        }
    })
}