Database
[{"_id":ObjectId("5b3f4e1013aad111501b34c4"),"Text":["kjsdf sdfjgnks vk ekl sdsdd."],"Title":["lkdjf wufk"],"List":["3412","1244","7654","8476"],"__v":0}]
Query
db.collection.aggregate([{$addFields:{totalList:{"$size":"$List"},List:{$substr:[{$reduce:{input:"$List",initialValue:"",in:{$concat:["$$value",",","$$this"]}}},1,50]}}},{$project:{List:1,Test:1,Title:1,totalList:1}}])