[{_id:"52cdef7c4bab8bd675297d8b",name:"AdventNet",Offices:[{Description:"Headquarters",city:"San Francisco"},{Description:"Headquarters",city:"San Francisco"}]}]

db.collection.aggregate([{$match:{$expr:{$eq:[{$size:{$filter:{input:"$Offices.city",cond:{$eq:["$$this","San Francisco"]}}}},2]}}}])