Your current life count is shown in the heart in the upper righthand corner of the Games list. Each time you start a new match it costs 1 life. As of version 1.0.5, lives will not automatically regenerate.
Additional lives can be purchased in the shop or you can watch a video ad to earn 1 life per video ad view (subject to availability). There is not a limit to the number of lives that can be stored. Infinite lives can also be purchased in the shop. Forfeiting a match does not recover a life.
If the video ad option is unavailable, you may just need to wait for the app to cache a new video ad. Ensure that your device has a good internet connection and check back again later. The app needs to be open to download a video ad but it will do it in the background as you play your matches or just have the app open. You don't need to sit in the shop screen. Video ads are not guaranteed and are subject to availability from the service we use. We are also adding another ad network with the next update that should improve video ad availability. Also in the next update, we are adding another way to earn lives by inviting your friends to play.