[{"_id":1,"beers":[{"brand":"Kozel","count":12},{"brand":"Pilsner","count":1}]},{"_id":2,"beers":[{"brand":"Kozel","count":3},{"brand":"Branik","count":11}]}]

db.collection.aggregate([{"$unwind":{"path":"$beers","preserveNullAndEmptyArrays":true}},{$group:{_id:null,count:{$sum:"$beers.count"}}}])