What happens when you have multiple copies of Nine Lives on the board?
Nine Lives second ability is a prevention effect not a triggered ability:

CR 615.1a Effects that use the word “prevent” are prevention effects. [...]

When there are two Nine Lives on the battlefield, the interactions are resolved according to:

CR 616.1. If two or more replacement and/or prevention effects are attempting to modify the way an event affects an object or player, the affected object’s controller [...] or the affected player chooses one to apply [...]

In this case you would choose on which copy of Nine Lives to put the counter, effectively giving you 17 lives.