If you need to identify the intruder in court, you are going to need a fairly detailed image, like 60ppf in ideal lighting conditions. See our identfication vs detection tutorial on that.
The next issue is how 'far' a speeddome can optically zoom and deliver that level. Roughly speaking you are looking at ~100 meters. See our 'How Far Can a PTZ See?' tutorial.
That implies putting a speeddome every ~200 meters.
However, this presumes no low light / night time issues. Unless this perimeter is very well lit or you do not care about night time monitoring, this is going to be a big problem as it will significantly cut down how far you can identify a person.
Finally, as a point of reference, if you only needed to detect a person and verify that one was there, the PTZs could cover ~6 greater range - a huge distance. This is mainly noted to emphasize how much of a tradeoff exists in requiring identification.
If you are willing to forgo a speeddome and use a pan / tilt positioner instead, there are some more possibilities that might get you far longer range with identification. See our recent discussion on 'Viewing a Person's Face 3000 meters away'. While that is not terribly realistic, you should be able to get to 500 meters with a specialized solution which could decrease the total number of cameras used by 5x (though each camera will be significantly more expensive).
Couple other things to keep in mind, beyond the bare math of viewing range:
2. The lower you mount the camera, the more likely you'll have obsctructions - for wide, sweeping coverage areas, you usually want to mount higher.
2. The higher you mount the cameras, the worse the angle for a clear facial shot (unless you can ID people by their bald spots).
3. As you mount cameras higher, the more the ACTUAL distance to a target on the ground increases.
Just a few things to factor into your design :)