Unfortunately I need to let you know the previous answer is not exactly correct. In fact it would be impossible to determine the amount of posts you would need without knowing the perimeter measurements of the property. At least fairly close. As an example if the property were 1000 feet along the front and back and 218 feet along the sides the size is quite close to 5 acres. Or 2436 lineal feet. This configuration would take 406 posts plus a few more for gates.
If your 5 acres was shaped like 500 feet at the front and the back and then 436 along the sides the lineal footage is only 1572 feet. Which would only take 312 posts plus a few extra for gates.
As you can see the perimeter layout is very important to determine how many posts would be needed. Without this no answer will be correct.
Note: Hold off on selecting the "accept" button if you have additional questions. If this answer works for you go ahead and make the "acceptance" and you may then, also use the "feedback" feature. With this you may leave input on the quality of my answer. This JA Web Site uses this input to evaluate it's contributing "experts" on an on going basis. Thank you in advance for doing this.. Dennis Dennis (cut2it)