There are two types of revives. One is friend-revive, which uses your friends’ ninjas or the strangers’ ninjas to continue the run; the other is self-revive, which is to run with your unlocked ninjas. Please note that using self-revive will cost you some jades.
Friend-revive can be used only once per run. If there is very few friends’ ninjas to choose, you may have the chance to choose the strangers’ ninjas as friend-revive at the cost of 100 coins per run. If the ninja of friend-revive dies, you can still continue the run with your unlocked ninjas. The number of times you can continue with self-revive is determined by your ninja Title. You can find out more information about the times you can revive by tapping the “Title” screen from the Home screen.
For example, if you are in the Title of Initiate, you can only revive with one of your unlocked ninjas. This means that you need to have at least two ninjas, one to run during the first life, and another to continue as self-revive after the first life. If you have only one ninja, you cannot do the self-revive because you do not have an extra unlocked ninja to continue the run.