Database
[{startDate:ISODate("2023-04-01"),endDate:ISODate("2023-04-10"),},{startDate:ISODate("2023-04-05"),endDate:ISODate("2023-04-07"),},]
Query
db.collection.aggregate([{$addFields:{dateDiff:{$divide:[{$subtract:["$endDate","$startDate"]},86400000]}}},{$addFields:{daysActive:{$map:{input:{$range:[0,"$dateDiff",1]},in:{$add:["$startDate",{$multiply:["$$this",86400000]}]}}}}},{$unwind:"$daysActive"},{$group:{_id:"$daysActive",members:{$sum:1}}},{$sort:{_id:1}}])