simple approach to limiting how many AudioSources play a specific clip in Unity

AudioVoices.cs

/*
* Usage:
* 
* in some other script create an AudioVoices object
* 
* public AudioVoices voiceLimiting = new AudioVoices();
* 
* then when playing a sound first add the AudioSource to the AudioVoices
* passing the maximum amount of instances and only play it if the source got added:
* 
* bool wasAdded = voiecLimiting.AddActiveSource(source, 5);
* if(wasAdded) source.Play();
* else Debug.LogWarning("coud't play " + source.clip + "! voice limit reached!");
* 
* Don't forget to keep calling the following every frame to make sure when sounds stop
* that new instances will be allowed to play:
* 
* voiceLimiting.CleanSources()
* 
*/

using UnityEngine;
using System.Collections.Generic;

public class AudioVoices {
    private Dictionary<AudioClip, List<AudioSource>> activeSources;
    private List<AudioClip> activeClips;

    public AudioVoices() {
        activeSources = new Dictionary<AudioClip, List<AudioSource>>();
        activeClips = new List<AudioClip>(8);
    }

    /// <summary>
    /// This should be called every now and then? Maybe per frame
    /// </summary>
    public void CleanSources() {
        for(int clip = activeClips.Count - 1; clip >= 0; clip--) {
            AudioClip key = activeClips[clip];
            List<AudioSource> list;
            if (activeSources.TryGetValue(key, out list) == false)
                continue;
            // iterate over the list and check isf sources have stopped
            for (int source = list.Count -1; source >= 0; source--) {
                if (list[source].isPlaying)
                    continue;
                // the source is not playing anymore. remove it from the list.
                list.RemoveAt(source);
                if (list.Count > 0)
                    continue;
                // clean up so less iteration happens when no source attached to a clip anymore
                activeSources.Remove(key);
                activeClips.RemoveAt(clip);
            }
        }
    }

    /// <summary>
    /// Add a source to the lookup dictionary if not exceeding maxCount
    /// </summary>
    /// <param name="source">the source to be added</param>
    /// <param name="maxCount">the maximum amount of sources for this audio clip</param>
    /// <returns></returns>
    public bool AddActiveSource(AudioSource source, int maxCount) {
        List<AudioSource> list;
        if (activeSources.TryGetValue(source.clip, out list) == false) {
            // can't add. no active audiosources allowed atm
            if (maxCount < 1)
                return false;
            // clip had no active sources. make a new list
            list = new List<AudioSource>(1);
            list.Add(source);
            activeSources.Add(source.clip, list);
            return true;
        }
        if (list.Count + 1 > maxCount) {
            // TODO: you can instead stop an older sound here if you want?
            return false;
        }
        // add the sound to the list
        list.Add(source);
        return true;
    }
}