How to structure your Ad groups?
The most important thing is to have only few keywords in an ad group. As you can read in the section "PPC Campaign Optimisation", you need to have your keywords included in your ad text (to have the opportunity to have them highlighted) and increase the relevancy of your ad in relation to the keywords.
If you put a hundred of keywords in an ad group, your ad text will not contain any of this keywords for a request, and your ad won’t return. In this case, the keywords in this ad group will have no impression and no traffic, and the reason is simply that your ad is not relevant for the keyword terms.
We usually put two main words in an ad group and decline those two words by using the long tail (1 or 2 other related attribute of those twp keywords), reversing the order of those words (if use Phrase match), or misspelling. At the end, if you follow this technique, you can obtain a list of 10 to 20 keywords phrases in an ad group.