Maximum 4 ad extensions can show for a particular query or device at any given time.
When extensions show, an ad is more prominent and, typically, performs better—that is, more people click on it. Extensions appear with ads on the Search Network.
“In each auction, we’ll generally show your highest performing and most useful combination of eligible extensions and formats. You will not be able to get a combination of extensions which gives more expected click-through-rate (CTR) than the expected CTR of a higher ad position”.