Just to add to what I I said above, check out the PTZ reviews on here. They should point you in the right direction.
I'm now going to try to explain a very dificult to grasp concept (I hope I have this right (no doubt others on here will flog me with a large branch if I get this wrong), and it will aid your undertsanding of why number of pixels isn't king besides the effect on low light visibility).
PTZ's have a lot of drawbacks but also 1 major advantage over fixed cameras and that is the number of pixels is less critical to getting a good picture because the definition of the pictue is defined by the resolution of the subject not the number of pixels in the overall frame. eg. take a person on the ptz at wide angle (or a wide angle fixed focal length camera) before the ptz zooms in, and who's far away and who takes up 1cm squared of the picture. For the sake of argument using nonsense numbers, lets say that 1cm is 10 pixels by 10 pixels. The person in the picture thus has a resolution of 100 pixels (10x10). Trying to enlarge the person in the image after the recording event, results in a poor picture because the pixel density is low - you only have 100 pixels and if you make it 10x larger in post recording, then you reduce the pixel density massively as magnifying it 10x makes the square 10x10 = 100cm2 but the captured resolution remains 100 pixels. So previously it was 100 pixels over 1cm squared, enlarged it becomes 100 pixels over 100cm squared. So pixel density drops from 100 pixels per cm squared to 1 pixel per cm squared, and that's why the subject cannot be identified by zooming into the recorded picture. With a fixed camera therefore, it's a compromise between resolution and sensor size because for a wide angle with a subject further away, the resolution and thus pixel density becomes the limiting factor in enlarging the picture to identify someone. Hence why people far from the camera in a wide angle shot, cannot be identified usually. If you add more resolution, (or a longer focal length to give more zoom), then you can capture people further away, but you pay for it with the limited field of view as the lens view is much narrower and extends further away from the camera, or in the case of resolution, you pay for it with low light visbility as more pixels on the same size sensor means less light captured at night and also the focus point of a fixed camera may also be a limiting factor irrespective of the number of pixels captured. So with more pixels, you can enlarge better but miss everything close up that's not centre of the picture, plus even then you probably won't be able to get a high enough resolution to compesate for the enlargement factor, you'll lose night visibility and irrespetcive of increased resolution, you may become limited by the focus point. eg if the fixed lens focuses at 16 feet and the subject is at 100 feet, irrespective of the number of pixels, they will be out of focus!
This is where the PTZ "cheats". It gives the best of both worlds. At wide angle you see everything, so don't need a huge resolution as it serves as an over view and very close up will have enough pixel density anyway - many are 2mp or 4mp for this reason. 2mp are for this reason, still widely used. However, when it spots someone far away and if the rules set trigger it, it will optically zoom into them. Optical zooming is different to the digital zooming (the enlargement) of the picture you do once it's recorded and viewing it back, in that you are effectively changing the lens on the camera to one that sees and focuses far away. In my example above, the 1cm square zoomed out is still 100 pixels. However, if the PTZ camera focuses on the subject and optically zooms in, it makes the subject fill the picture more. The resolution of the overall picture stays the same, but the resolution of the recorded square is increased as it fills more of the frame. So whereas before it might have been 1cm squared and 100 pixels, in the optical zoom if it now fills 90% of the frame and the frame contains 2mp (2,000 pixels). The subject ie the square now contains 90% of 2,000 = 1,800 pixels. So it not only appears larger but is sharper as it has a higher pixel density because it fills more of the frame. In addition, the optical zoom on a ptz adjusts focus to lock on the subject so the ptz is always at the perfect focal point whereas even if within the focal range of a fixed lens camera, the focus related sharpness may drop off towards the outer limits of the in focus range and will drop off outside this rapidly. So even though you may only 2mp, the same square subject which is 100 pixels AND unsharp because outside the focus range, may be sharp as perfectly focused on the PTZ as it adjusts the focus to match, and may be in this example 1,800 pixels in resolution on a 2mp sensor vs 100 pixels for eg an 8mp fixed wide angle camera where it is enlarged in viewing post. For that reason, you will see more successfuly at distance with eg a 2mp PTZ than an 8mp wide angle fixed. Mnay now use 4mp on ptz but again, watch sensor size if you want a good night picture. You may also have to pay a lot more to get colour at night as opposed to B&W and IR on the ptz.
Difficult to explain so hope I got that right and it doesn't confuse. Suffice to say, the short answer is, a fixed focal length camera has a useable range in which a subject can be in focus, have sufficient pixel density and be recognisable. Conversely, with a PTZ, even a smaller sensor will often produce a picture in which someone who's far away can be indentified if it triggers and optically zooms into them. PTZ's have their own issues which is why many choose fixed focal length and may use multiple cameras of different focal lengths covering the same area to give them both distant and near recognisability. However, it's beyond the scope of what I'm going to say here beyond that they can have issues locking onto the subject and following them or selecting the right subject without cues from another camera where multiple subjects are in view.
