Rengar's abilities have no cost related to them and their use is only limited by cooldowns. Instead he uses Ferocity as his secondary resource.
Rengar gains 1 point of Ferocity with each ability he uses on enemies (including structures and neutral monsters). Upon reaching 5 Ferocity Rengar gains the ability to cast an alternate set of abilities, which are empowered versions of his basic abilities. Using an empowered ability consumes all 5 Ferocity. The two ability-sets do not share cooldowns, and his basic abilities will continue to count down while his empowered abilities are available. His empowered abilities have no cooldown and can be used any time 5 Ferocity is available.