Kalki avatar would be one and only one, the annihilator of present Kali Yuga (the Dark Age). Despite all scientific advancements the present age is called Dark Age because it is ruled by material tendencies rather than spiritual.
Primarily all living beings including human beings are spiritual from the beginning of life. We are a spirit, a soul atman manifesting human form. Our ultimate goal of life also is spiritual, becoming a pure soul atman, emancipating self from cycle of birth and death forever.
Avatars in future would be many but Kalki avatar would be one and only one similar to Lord Krishna. There would never be another Lord Krishna in future ever.
As ordained by God life comes full circle every 3500 to 5000 years and in the end stages of a Yuga (era) precipitates an avatar to re-establish Dharma (righteousness).
After advent of Kalki avatar in about a decade from now the next avatar would advent after about 3500 to 5000 years. The best part of life is mankind would witness an avatar in present times. An ordinary folk, one amongst us would don supernatural powers to become an avatar, the awaited Kalki avatar!