AFAIK Google doesn't reveal data about its advertisers (not to small and medium-level advertisers, not sure about the rest ) There are several paid BI tools such as KeywordSpy, SpyFu, PPCBully and more. They sample Google SERPs for an enourmous list of keywords and offer this data for a few $100s/month. You can look for tools' free trials, but remember that this is 2nd-hand data, i.e. not very accurate.
As natively said you can use spyfu to see how many advertiser are for your specific keyword. It`s a good tool and free to use. Try it and you are going to find out what you are looking for.
Just to clarify - I'm not promoting any of those tools. In fact, for me it wasn't worth the cost, including SpyFu. For etrieving some basic data, like a list of competitors for a few search terms, you can use the free trial. And even with the most profound keyword research and the most comprehensive tool - you won't really know anything important until you launch the #@$% campaign