Database
db={"collection1":[{"_id":"user1"},{"_id":"user2"}],"collection2":[{"name":"test","Info":{"initiatedBy":"user1"}},{"name":"Test LFE","Info":{"initiatedBy":"user1"}},{"name":"Test uuE","Info":{"initiatedBy":"user2"}}]}
Query
db.collection1.aggregate([{$lookup:{from:"collection2",localField:"_id",foreignField:"Info.initiatedBy",as:"matchedRecords"}},{$project:{_id:1,count:{$size:"$matchedRecords"}}}])