From Halopedia, the Halo wiki
At least 6
Unified Earth Government
The Beta Hydri system is in the constellation Hydrus. It is about 24.4 light years away from Earth, and its star is larger and slightly greater in magnitude than Sol.
By the 26th century, at least six planets had been discovered in this system and humanity had begun colonization of the stellar region. One of the system's colonized planets was Beta Hydri VI.