[{fullName:"Vehicle 1",tags:["car","bike"],},{fullName:"Vehicle 1",tags:["car"],},{fullName:"Vehicle 1",tags:["unknown"],}]

db.collection.aggregate([{$unwind:"$tags"},{$match:{tags:{$regex:"car",$options:"i"},}},{$group:{_id:"$tags",count:{$sum:1}}},{$project:{name:"$_id",_id:false,count:"$count"}},])