Hi, maybe a stupid question, but where can I get this double black wire in the picture?

 

Hello,

As far as I can tell, the black wires connect the GND connections of the boards.
Just connect all the points of the black wires together.

Bertus

 

Local hardware store. Assuming you want 16 or 18 AWG. You didn't specify.

 

Hi, maybe a stupid question, but where can I get this double black wire in the picture?

The colored wires in the stupid cartoon just mean that those points are connected together. They don't imply any the use of any "special" wires.

This is how it could have been drawn:

 

The colored wires in the stupid cartoon just mean that those points are connected together. They don't imply any the use of any "special" wires.

This is how it could have been drawn:
View attachment 210301

Is there any limit how many points can be connected together?

 

Is there any limit how many points can be connected together?

No.

 

Is there any limit how many points can be connected together?

No.
Mechanically, it may be harder to connect many together at one terminal, but then for a lot of wires, have maybe 4 per terminal, with a wire looping between terminals that are connected together.

 

Is there any limit how many points can be connected together?

In theory, no.

Practically, it depends on the current flowing through the wires. Wires have resistance, so there's a limit to the current they can carry and there will be a voltage drop in the wires. This isn't an issue with sufficiently low currents and/or sufficiently low resistance connections.

Nothing in electronics is ever as simple as you'd like it to be.

EDIT: If whoever drew that wiring diagram had given it a little more thought, it could have been drawn more along the lines of this to minimize wire crossings and routing length: