[{"_id":1,"user_access_type":["Mobile","Card"]},{"_id":2,"user_access_type":["Card","Mobile"]},{"_id":3,"user_access_type":["Mobile"]},{"_id":4,"user_access_type":["Card"]}]

db.collection.aggregate([{$addFields:{user_access_type:{$setUnion:"$user_access_type"}}},{$group:{_id:"$user_access_type",count:{$sum:1}}}])