[{"_id":"1_0","data":[{"Class":"DUMMY1","mark":"5"}]},{"_id":"2_0","data":[{"Class":"DUMMY2","mark":"3"}]},{"_id":"3_0","data":[{"Class":"DUMMY3","mark":"9"}]}]

db.collection.aggregate([{$project:{adjustedGrades:{$map:{input:"$data",as:"grade",in:{$lte:[{$toInt:"$$grade.mark"},5]}}}}},{$match:{adjustedGrades:{$eq:true}}},{"$count":"count"}])