At least 2
"The zerg first evolved here, yes."
"I do not like it. A soft world. Char is far superior."
"Don't be so sure. Zerus may prove more dangerous than any world we've known."
The first zerg arose from the first spawning pool, where one essence split into many and one devoured another and became stronger. The zerg were the most insignificant life form on Zerus until the proto-genetic manipulations of the xel'naga enabled them to survive the planet's torrential firestorms and thrive. Amon desired their ability to steal essence, but the zerg were independent, so he bound them to the hive mind and gave them a directive to destroy the protoss, and accordingly, the Swarm departed for Aiur. Some zerg, including Zurvan, were overlooked and became the primal zerg.
Dark Prelate Zeratul visited Zerus and returned with Sarah Kerrigan, who proceeded to awaken Zurvan by harvesting quillgor meat and feeding it to him. However, Brakk declared that she intruded on his territory and sent his pack to destroy the meat. When this was unsuccessful, he personally led an attack on her hive cluster and was killed.
On Zurvan's instructions, Kerrigan entered the first spawning pool. Yagdra claimed Brakk's territory and his pack's essence. The Swarm defended Kerrigan's chrysalis from both Yagdra's pack and the remnants of Brakk's pack, who engaged each other in a basin south of the first spawning pool. Yagdra won, but failed to prevent Kerrigan's transformation into the Primal Queen of Blades.
Yagdra, Kraith and Slivan each sent most of their pack against the Swarm, leaving themselves vulnerable. With help from Dehaka's pack, Kerrigan killed each of them and their packs joined her Swarm. Kerrigan's essence called Zurvan and he turned on her. Kerrigan killed Zurvan and Zerus became hers. Dehaka and his pack accompanied Kerrigan on her return to the Koprulu sector.
Blizzard ran out of time to dedicate artistic support to Zerus for Heart of the Swarm, It was reasoned that if it came to a choice between fleshing out the zerg and Zerus or Kerrigan, moving Kerrigan's story forward had to take priority. This included the apparently fast travel time for Kerrigan's leviathan and the Void Seeker to the world—it was not intentional to give the impression that Zerus was in easy range of the races of the Koprulu sector. However, it was reasoned that time and scale was not something that everyone appreciates.
Discrepancies exist between lore for StarCraft and Heart of the Swarm as to how Zerus is portrayed. The manual of the original game states that the zerg assimilated every lifeform on Zerus within a few centuries, and left it behind as a lifeless, burning rock. Heart of the Swarm depicts Zerus as a verdant jungle world with fauna present.