[{_id:ObjectId("632c12d98b4d9347774a4e9f"),activeStartDate:ISODate("2022-09-02T23:18:10Z"),activeEndDate:ISODate("2022-09-28T23:47:10Z")},{_id:ObjectId("632c12d98b4d9347774a4e9e"),activeStartDate:ISODate("2022-09-29T23:18:10Z"),activeEndDate:ISODate("2022-10-06T15:47:10Z")},{_id:ObjectId("632c12d98b4d9347774a4e9d"),activeStartDate:ISODate("2021-04-26T23:18:10Z"),activeEndDate:ISODate("2022-09-28T22:44:13Z")},{_id:ObjectId("632c12d98b4d9347774a4e9c"),activeStartDate:ISODate("2022-04-01T10:12:11Z"),activeEndDate:ISODate("2022-10-01T13:57:01Z")}]

db.collection.aggregate([{$set:{months:{$map:{input:{$range:[0,{$add:[{$dateDiff:{startDate:"$activeStartDate",endDate:"$activeEndDate",unit:"month"}},1]}]},in:{$dateAdd:{startDate:{$dateTrunc:{date:"$activeStartDate",unit:"month"}},unit:"month",amount:"$$this"}}}}}},{$unwind:"$months"},{$group:{_id:{year:{$year:"$months"},month:{$month:"$months"}},activeCustomers:{$sum:1}}}])