As said above, maps are not safe inside go routines like this.

The simplest way to modify your example is to add a mutex and perform lock/unlock on each pass.

package main

import (
    "fmt"
    "sync"
)

type Counter struct {
    m  map[string]int
    mu sync.Mutex
}

func main() {
    counter := Counter{m: make(map[string]int)}
    var wg sync.WaitGroup

    for i := 0; i < 100; i++ {
        wg.Add(1)
        go func(id int) {
            defer wg.Done()
            key := fmt.Sprintf("key%d", id)

            counter.mu.Lock()
            defer counter.mu.Unlock()
            counter.m[key] = id * 10
        }(i)
    }

    wg.Wait()
    fmt.Println(counter.m)
}

This should help demonstrate the issue. But there are other ways to handle this, with read/write mutex or a channel. Note that using locks may slow down the program, compared to other methods.

More discussion: How safe are Golang maps for concurrent Read/Write operations?