Welcome Steve
The single CAT6 per camera is likely all you will need. The additional wiring in a Dahua wiring harness/pigtail are mostly legacy connectors for integrating into existing analog camera systems and can be ignored in new builds like yours.
As others have mentioned run additional CAT6 cables to each location. One of the common reasons is to use the additional free CAT6 to power a PoE or 12V separate iR illuminator. This gives you an off axis iR light source which can bring up greater detail in the cameras capture, reduce or eliminate retroreflector glare, output substantially more iR light than built in emitters and eliminate spiders building their webs over your camera lens. You could also use additional CAT6 to place WiFi access points from companies like Ruckus Wireless around the house and property giving a seamless WiFi experience no matter where you are on the property or how many guests you have visiting. Ruckus WiFi access points are also PoE capable.
Built in camera microphones are usually passable at best. For true audio recording with any real clarity and functionality beyond a few feet/meters you would need separate dedicated audio hardware setup which uses yet more CAT6 cables. Additionally be very sure to check your jurisdictions laws regarding audio recordings, they can vary widely between the different levels of government you reside in and can carry very serious consequences if violated. Admittedly I have not experienced audio recordings from all models of Dahua and perhaps there are units will very good audio recording plus the installation geometry and a number of other factors can have a significant impact on audio quality I'm sure.
Having conduit with pull lines installed to the CAT6 cable locations can also be very valuable. It would give you as much "future proofing" as is possible by allowing you do easily pull new cables in the future should the need arise. This could include additional CAT6 or some hitherto unknown future cabling which hasn't been invented yet. Just imagine how homes built in the 60s and 70s could never have imagined how many power outlets a modern home needs or data cabling to every room.
Think of varifocal cameras as adjustable wrenches, they are suitable across a variety of implementations and uses. Think of a fixed focal camera just like a fixed size wrench, if it happens to fit the bolt you are good to go but it is doesn't it is nearly useless. This is primarily why most everyone on this site recommends varifocal cameras, they have a degree of flexibility that affords them the ability to be useful in many different scenarios. Most commonly if the camera location has a substantial amount of blank wall in the view a varifocal can zoom in to reduce that while a fixed does not. The wall will likely reflect iR light at night and cause the visible wall to be very bright and the rest of the scene very dark.
One thing to pay close attention to when choosing your cameras is NOT all cameras support PoE (Power Over Ethernet) which means you would need a separate 12v power line as well. There are baluns available that you can use on a separate CAT6 cable to supply 12v power. Additionally take into consideration both the different PoE specs your switch supports and the switches total PoE budget. The higher end the PoE switch usually the higher the PoE budget. The two common PoE specs are IEEE802.3at and IEEE802.3af also known as PoE and PoE+ respectively. PoE can supply 15.4W and PoE+ can supply 25.5W. Typically fixed and varifocal cameras use PoE while PTZ and other large cameras often use PoE+. The switch PoE power budget is a fairly simply concept, a hypothetical PoE switch that has 48 ports with PoE+ and PoE, all ports have PoE and the first 8 ports have PoE & PoE+, the switch has a total PoE budget of 200W. The total PoE draw for the switch across all ports cannot exceed 200W. So if you have 48 cameras connected using PoE that each only draw 3W you are fine but if you connected cameras that draw 5W of PoE you can only connect 40 cameras not the full 48. This is an over simplification but you get the idea. Cameras with iR turned on draw more power than iR turned off which means cameras can draw less power during daylight and more at night.
Choosing a central location for all of these cables to run to is also something to think about. Most all of the equipment at this central hub or server room or rack, whatever you intend to build or call it, will generate heat as a byproduct. Small common home implementations tend not to have enough equipment for this to be taken into consideration but as you as appear to be adding many cameras you might need to start thinking about this. The equipment you use in your hub can also generate noise in the form of fans turning and hard drives spinning, some equipment is utterly silent while others can be very loud. If you live in a hot environment it is usually best to place the equipment within the confines of the climate controlled interior of the home as heat is the most common enemy of electronics. Placing equipment in a non-climate controlled garage can shorten that equipment lifespan, degrade performance and force the generation of additional noise by fans turning faster to try to cool the equipment.
Also consider the physical security of the equipment and your recorded footage. If a burglar can remove all the recorded footage the cameras are less valuable to you after the fact. Some Dahua cameras can accept a microSD card and record footage to it in addition to your chosen NVR such as a Dahua brand or
Blue Iris PC etc.
