How to display Interstitial Ads after a set number of plays

I have integrated Chartboost to my project and I am trying to display an ad when a user loses. I have created a CreateAd method in a script and I am calling it whenever the user loses and gets the game over screen.

public void CreateAd () {
		CBBinding.showInterstitial(null);
	}

But I want to count how many times a user has played in a single sitting and display an ad every five plays or so. Not every time. Also could someone please show me how to check if Chartboost is available and if not use a different ad service like Admob or inMobi?

First answer as posted by rutter. I use to display add every x games unless player reached some minimum result

playCount++;
if (playCount % 5 == 0 || result>1000) {
    //do something
playCount = 0; // reset the counter
}

As for the second part, chartboost api has the flag hasCachedInterstitial

if(CBBinding.hasCachedInterstitial(null))
{
   CBBinding.showInterstitial(null);
}

You can also listen to delegate
public static event Action didCacheInterstitialEvent;

which is fired once ad is cached.
more info about it here
Chartboost unity manual