It's great practice to use multiple ads in your campaign. We recommend 4-5 until you have a good idea of what's working for you. You can always add or remove ads from your campaigns. That being said, it is possible to have too many ads in one campaign!
If you are using a lot of ads in your campaign, you may find that your campaign is serving few or no impressions. The number of ads may be why. If your budget isn't large enough to allow enough funds to allocate to your ads to be competitive, you may not win the ad auction.
Example: You have a campaign with 30 ads, and a $25/week budget.
Our billing system is designed to take your weekly budget and divide it by 7, giving you an even amount of money to spend each day. With a $25/week budget, the campaign allocates roughly $3.57/day to spend on your ads.
From there, the system will allocate an equal amount of funds per ad to use for bidding in ad auctions. In my example, that means each ad only has about 11 cents to use for bidding! ($3.57/day divided by 30 ads = $0.11)
The ads are all being outbid, so the campaign isn't serving. To solve this, remove some of the ads, or greatly increase your budget!
Using many ads not only can cause problems with the ads serving, but it can also make it very difficult and time consuming to determine what ads are working for you and what isn't, because your impressions are being spread so thin. Keep your campaigns to 10 ads or less until you have a solid idea of how things look.