Golang Chan＆Waitgroup有时会等待很长时间？

I'm new golang. I write a parallel request code but sometimes occur wait a long time. Do you know any reason?

var wg sync.WaitGroup
    guard := make(chan struct{}, 50)

    for _, li := range results {
       wg.Add(1)
       guard <- struct{}{}
       go func(){
        defer func(){//I'm afraid some panic, so I use defer func, make sure it has done.
            wg.Done()
            <-guard
         }()
          //some biz code 
          //http requests
       }()
    }
    wg.Wait()

I try print process number, observer number, always stuck the last process long time. I don't know the stuck reason. It can be finished, just stuck a long time in the last process.