var wg sync.WaitGroup
// Create bufferized channel with size 5
goroutines := make(chan struct{}, 5)
// Read data from input channel
for data := range input {
    // 1 struct{}{} - 1 goroutine
    goroutines <- struct{}{}
    wg.Add(1)
    go func(data string, goroutines <-chan struct{}, results chan<- result, wg *sync.WaitGroup) {
        // Process data
        results <- process(data)
        // Read from "goroutines" channel to free space for new goroutine
        <-goroutines
        wg.Done()
    }(data, goroutines, results, &wg)
}

wg.Wait()
close(goroutines)