[{_id:1,count:[1,2,3]},{_id:2,count:[4,5,]}]

db.collection.update({},[{$set:{count:{$cond:{if:{$gte:[{$size:"$count"},3]},then:{$concatArrays:[{$slice:["$count",0,{$subtract:[{$size:"$count"},1]}]},[10]]},else:{$concatArrays:["$count",[10]]}}}}}],{multi:true})