WanWizard

Does that mean that 1 FBC module, that have 2 inputs  = 16 (2x 8) tuners ?  and with 2 FBC modules = 32 tuners

No. 1 FBC module = 2 root tuners (each with an input), and 6 leaf tuners (looped through on one of the root tuners), makes 8 tuners per FBC module.
 

What did you mean with "providing your setup can provide 8 SCRs"

In a DiseqC setup, the entire frequency spectrum recieved by the LNB is send to the tuner, and the tuners tunes on a transponder frequency in that spectrum. Which is why you are limited to one per cable, no matter how many tuners you loop through on it.

In a Unicable setup, the LNB does the tuning, and then maps the transponder to a userband or SCR. There can be up to 32 userbands per cable, which is why you can connect up to 32 tuners to a single Unicable cable. How many userbands are actually available depends on the LNB and/or switch setup.
 

I will buy a new "One cable" LNB with with as many "User bands" as possible.  I know that there are LNB's with 3 outputs,  2x "old type" LNB output + 1 "Sat CR" output with 24 "Userbands"/Channels.

If you only need to receive one satellite position, that is an option. 
 

Using one cable from the Sat CR  output on the LNB and at the other end of the cable (in my living room) I use a 4-way splitter and connect to each of the four inputs on the FBC modules. Two per module.

Possible, make sure those splitters are Unicable splitters, simple ones don't work.
 
Is that enough except for the configuration in the box? or do I need some more hardware?
 

The Sat. Box will probably bee Vu+ Ultimo4K or Duo4K, with 2x FBC DVB-S2x modules.

You're expecting more than 8 transponders simultaneously in use on a single box?

Precies. De DM800SE is bij ons end-of-life, er is geen recente OpenPLi voor beschikbaar.

Het zou wat wezen als dat niet zo kunnen...

Misschien even uitleggen wat je precies doet?

You have to be careful with terminology, and also take the environment into account, as it is very easy to misunderstand how things work, and misinform others who read it.

When you start with a tuner, a tuner can receive a single transponder at the time. That is true for all tuners, S, C and T.

From this it follows that if you have a module with two tuners (in marketing called dual tuners or twin tuners), you can receive two different transponders at any given time. So contrary to your statement, such is module IS identical to two single tuners.

However, if you have TWO S tuners connected to ONE cable, both tuners will be restricted to what can be transported over that single cable, which in a DiseqC setup is a single LNB quadrant, theoretically 1/4th of the available transponders. If you have a Unicable setup, you can receive two transponders, even if you have only a single cable.

If you have an FBC C module, you have 8 tuners connected to one cable, and the possibility to simultaneously receive 8 transponders. I haven't seen an FBC T module yet.

For S, it is a bit different. An FBC S module has 2 root tuners, connected to each of the inputs, and 6 leaf tuners, which are dynamically looped through on one of the root tuner inputs.

If you have a DiseqC setup, both root tuners will behave identical to a dual/twin tuner module. The 6 leaf tuners will be able to tune transponders in either of the quadrants locked by one of the root tuners, identically to having a dual/twin tuner with a single cable. If you have a Unicable setup, all 8 tuners can tune to a different transponder over a single cable (providing your setup can provide 8 SCRs).

For both twin/dual tuner S modules and FBC S modules you can mix both setups, so you can have a DiseqC setup on one tuner, and a Unicable setup on the other one. Like you can with two single tuner modules.

BBC Three has been off air for ages?