Joshua 12 is a list of the kings defeated by the Israelites under the leadership of Moses and Joshua. Verse 17, "the king of Tappuah, one; the king of Hepher, one;" simply means that the Israelites defeated one king from the city of Tappuah and one king from the city of Hepher.

Here's a breakdown:

"the king of Tappuah, one;": This indicates that the Israelites conquered Tappuah and its ruler. Tappuah was a Canaanite city, likely a small city-state. "One" refers to the fact that they defeated one king from that specific city.
"the king of Hepher, one;": Similarly, this means the Israelites conquered Hepher and its ruler, defeating one king. Hepher was another Canaanite city-state.

Essentially, it's just part of a running inventory of defeated kings and their respective city-states. The list demonstrates the extent of the Israelite conquest of Canaan. It's important to remember that in this context, a "king" often refers to the ruler of a small city-state, not necessarily a king in the modern sense of ruling a large territory.