concurrency in go -Taking same time with different no of CPU

you will find out:

total time (3 min 20s) = 200s = sleep(200ms) * loops(1000)

Let's simplify your code and focus on CPU usage:

1. Remove the Sleep, which does not use the CPU at all
2. fmt.Println as a stdio, does not use the CPU
3. Random number did nothing but introduce uncertainty into the program, remove it
4. The only code that takes CPU in the goroutine is the "rand.Intn(20)+20", making it a constant addition
5. Increase the "maxRandomNums"

then your code will be like this, run it again

package main

import (
    "fmt"
    "runtime"
    "sync"
    "time"
)

var waitG sync.WaitGroup
var cpuUsed = 1
var maxRandomNums = 1000000000

func init() {
    maxCPU := runtime.NumCPU() //It'll give us the max CPU :)

    cpuUsed = 8 //getting same time taken for 1 and 8
    runtime.GOMAXPROCS(cpuUsed)

    fmt.Printf("Number of CPUs (Total=%d - Used=%d) \n", maxCPU, cpuUsed)
}

func main() {

    start := time.Now()
    ids := []string{"rotine1", "routine2", "routine3", "routine4"}

    waitG.Add(4)
    for i := range ids {
        go numbers(ids[i])
    }
    waitG.Wait()

    elapsed := time.Since(start)
    fmt.Printf("\nprogram took %s. \n", elapsed)
}

func numbers(id string) {
    // rand.Seed(time.Now().UnixNano())
    for i := 1; i <= maxRandomNums; i++ {
        // time.Sleep(200 * time.Millisecond)
        // fmt.Printf("%s-%d  ", id, rand.Intn(20)+20)
        _ = i + 20

    }
    waitG.Done()
}