I can't answer for those manufacturers but I can tell you that if you use third party PVR software such as Tvheadend, you can have as many HDHomeRun tuners as you want, and probably this would be true of tuners from other manufacturers as well. Your biggest issue will be splitting your antenna to feed all those tuners, although if you have some stations in one direction and some in another you could have two or more TV antennas, each feeding separate HDHomeRun devices. I've seen as many as three HDHomeRun dual tuner units (for a total of six tuners) feeding a single Tvheadend installation, and with Tvheadend at least you only need an additional tuner if you are tuning another physical channel, in other words if you are watching or recording from two different channels on the same physical channel (for example 10.1 and 10.2) then it will only use one tuner for both channels. Tvheadend runs on Linux but there are several other similar programs, some of which run on Linux and others which run on Windows. Some others I can think of off the top of my head are MythTV, MediaPortal, and NextPVR, though I am not familiar enough with any of those to know if they limit the number of tuners that can be connected. I don't see any reason they should, but I don't know.
Obviously stacking HDHomeRun devices can get a little pricey, so I'd at least wait until their ATSC 3.0 models are out, so you don't drop a bunch of money on devices that will soon be obsolete.
I’m adding a vote for Channels DVR. I’m running most of our house TVs on it now. In my case it runs on a Synology NAS and pulls feeds from all my HDHomeRun devices in. You also can log in using your DirecTV, Spectrum, etc accounts and it adds the cable channels that offer TVEverywhere streaming to your guide as well. It even allows you to record the TVE feeds and provides a real, traditional looking guide. It has commercial detection and can even automatically skip commercials when playing back on some devices.
I have 2 tablo tuners. 1 has 2 tuners and the other 1 has 4 tuners. The reason I have 2 players is I wanted to dedicate one to channels north of me and the other one south direction Use a different zip code for each tuner as your guide is determined by the zip codes you use. I am very satisfied with my tablo,s
Not enough (perceieved) market value to justify the cost.
Most people are assumed to watch live TV, and record a show or two at the same time, max.
When we did an an STB design for the national cable company here in 2014, I upped the specs from a two-tuner (one for recording, one for live tv) to an 8-tuner design (4 for FCC live tv, 4 for recording or "hopper" use).
The cost of the tuner was not that much more, it was a move from discrete tuners to Full-Bandwidth Capture (FBC) w. 8 digitial "taps" (tuners). The key driver for this was the user experience - our brief was to create the best viewing experience possible within the limitation of cable-tv. This meant using 4 tuners for live TV browsing, and 4 for recording.
While the design team was repeatedly challenged on the cost of the FBC tuner w. 8 taps, we prevailed. And had great reviews in the press when we launched.
Channel change time was almost instantaneous. Recording 3 programs at the same time covered 99.9% of needs of our customers. The box was (for these but also other reasons) perceived to be the by far "fastest" on the market.
At least for just for a few months, it was really greaty to be basking in the "fastest box in the country"-light. It sure was good to see that the intent of making the (almost) fastest experience possible came to bear fruit.
But the thing is - when you look at the cost of the box, then then 8 "tuners" added a few dollars more in HW cost.
Which is crucial to scale. Especially if 80% of your audience are happy with two "simple" tuners.
What more, though, is that the features enabled required more complex UI, and more complex software on the box, which in turn meant higher development, support and maintenance costs etc.
We were going for the "best-in-class User Experience". Not all companies have that luxury. Some only go for the "best price/value ratio".
So, for most ("average") tv companies relying on delivering live (or time-shifted live) tv, 8 tuners are expensive on many fronts, and scaling this to a few million customers is probably not worth it.
For enthusiasts, you can build a PC with multiple tuners cards, and have 12 tuners in a tv-headend (or equivalent) setting, but it will be a minority interest group.
So I think the answer to you question chimes in with the other posters - it's not worth it in the bigger picture (both HW cost and SW development and maintenance cost).