Since $C5$ is the competitor of each of the other companies, therefore the only competitor of $C1$ is $C5$.
Let’s consider $C3$: Number of competitors is $6$.
We know for sure that $C1$ is not the competitor of $C3$; therefore the six competitors of $C3$ are $C2, C4, C5, C6, C7$ and $C8.$
Similarly $C8$: Number of competitors is $6$ and the competitors of $C8$ are $C2, C3, C4, C5, C6$ and $C7$.
Therefore competitors of $C7$ are $C5, C3$ and $C8$ and competitors of $C2$ are $C5, C3$ and $C8$.
Since the competitors of $C1, C2, C3, C5, C7$ and $C8$ are known and fixed by us, the third competitor has to be $C6$.
Therefore the competitors of $C6$ are $C5, C3, C8$ and $C4$.
Now consider $C4$: Out of the four competitors it has, three are $C5, C3$ and $C8$.
Since the number of competitors of $C1, C2$ and $C7$ is $1, 3$ and $3$ respectively, therefore the number of competitors who sell furniture in the regions in which either of $C1, C2$ and $C7$ sell furniture can be at most 4.
So the region in which maximum possible numbers of competitors sell furniture is the one that sells $C4$ or $C6$.
So, such a region could possibly have $C5, C4, C6, C3$ and $C8$.
Hence at most 5 competitors can sell furniture in one region.