I would like to know if it is possible to control like 20 3.digit, 7-segment modules with one Arduino?
Or is there som extra electronics I need to make it happen?
Yes, you need MAX7219 modules - like this.
You can remove and re-mount the 4-digit displays as required. The extra digit comes for free.
Nice!
The Max7219 is a 2x4-digit module, and thats also okay for me.
But can I control 9 more of them with one Arduino?
Sure can!
They cascade, one into the next, and require only three Arduino pins to control one or many. Nine is easily do-able, (some people have had trouble with 10 or more; this is an electrical rather than a software concern) but you are (always) advised to take care with the 5 V supply wiring, while these use relatively little current, it may be advisable to have parallel runs of the 5 V and ground wires to connect into the chain every three or four modules. Also, these modules have a 0.1 µF capacitor on each but you arguably should add a 10 µF capacitor every second or third module. (There is a place for it on the PCB!)
Always keep the wiring bundled together, avoid open loops.
Here is the MAX7219 datasheet. There are libraries for it but it is easy enough to write your own code for it. You can use SPI or shiftOut function.
Thanks for a quick reply!