Database
[{"students":[["5c8783f0927b1849b923a47b","5c889c139af6b305e19cd17e","5c89f1dc4dc6695c138cb2a4","5cb0624945111f1c5fdf2527","5caed95afd229a5c460e745b","5cbeb926fac7143e261adaab"],["5c8783f0927b1849b923a47b","5c889c139af6b305e19cd17e","5c89f1dc4dc6695c138cb2a4"],["5c8783f0927b1849b923a47b","5c89f1dc4dc6695c138cb2a4","5cadc0452a00532a4c903c38"],["5c8783f0927b1849b923a47b"]]}]
Query
db.collection.aggregate([{$project:{students:{$map:{input:"$students",in:{$size:"$$this"}}}}}])