Each ad group can have multiple ads that will trigger the keywords within that ad group. The ads will rotate and either be displayed evenly or start to be preferred and optimized by AdWords, according to the settings you choose.
Here is what you'll need to consider when choosing ad rotation settings:
If you are not tracking conversions and would like to maximize clicks, choose the Optimize for clicks option. This is the default option when you create a new campaign.
If you are tracking conversions and your goal is to maximize conversions within a campaign, your best bet is to choose Optimize for conversions.
Are you A/B split testing ads within this campaign? If so, you should choose either Rotate evenly or Rotate indefinitely. A/B testing allows you to experiment and test the impact of using different ad elements, requiring an even number of impressions delivered to each ad.