Always have x number of goroutines running at any time

You may find Go Concurrency Patterns article interesting, especially Bounded parallelism section, it explains the exact pattern you need.

You can use channel of empty structs as a limiting guard to control number of concurrent worker goroutines:

package main

import "fmt"

func main() {
    maxGoroutines := 10
    guard := make(chan struct{}, maxGoroutines)

    for i := 0; i < 30; i++ {
        guard <- struct{}{} // would block if guard channel is already filled
        go func(n int) {
            worker(n)
            <-guard
        }(i)
    }
}

func worker(i int) { fmt.Println("doing work on", i) }