Answer:
990 deers is ONE possible answer.
Explanation:
Since we don't know anything else than the perimeter, we don't know the exact dimensions of the park.
For example, it could be 10 x 11 miles, perimeter is 42, but the area is 110 sq miles.
It could also be 9 x 12 miles, perimeter is again 42, but the area is now 110 sq miles.
It could be a 1 x 20 miles park, perimeter is again 42, but the area is just 20 sq miles.
And it could be many other dimensions scenarios. That affect of course the calculation result.
For this answer, I'll assume the park is 10x11 miles, for an area of 110 sq miles.
Since the estimation is that there's 9 deeps for square mile... that means that for a park measuring 10x11 miles, there would be 990 deers (9 * 110).