[{username:"jack",events:[{eventType:"party",createdAt:"2022-01-23T10:26:11.214Z",visitorInfo:{visitorId:"87654321-0ebb-4238-8bf7-87654321"}},{eventType:"party",createdAt:"2022-01-30T10:26:11.214Z",visitorInfo:{visitorId:"87654321-0ebb-4238-8bf7-87654321"}},{eventType:"party",createdAt:"2022-01-29T10:26:11.214Z",visitorInfo:{visitorId:"12345678-0ebb-4238-8bf7-12345678"}},{eventType:"partyg",createdAt:"2022-01-31T10:16:11.214Z",visitorInfo:{visitorId:"12345678-0ebb-4238-8bf7-12345678"}}]}]

db.collection.aggregate([{$match:{username:"jack"}},{$project:{p:{$filter:{input:"$events",as:"item",cond:{$eq:["$$item.eventType","party"]}}}}},{$unwind:"$p"},{$group:{_id:"0",visitors:{$push:"$p.visitorInfo.visitorId"},uniqueVisitors:{$addToSet:"$p.visitorInfo.visitorId"}}},{$project:{_id:0,visitorsCount:{$size:"$visitors"},uniqueVisitorsCount:{$size:"$uniqueVisitors"}}}])