Addressable like 7-segments number display
Can the controller "auto discover" the total number of addressable RGB LEDs?


Sure. You just need to connect the data output of the last LED back to the controller. If you want to create a stackable module, you'll need a loopback cap that goes on the far end. You can stack any number of digit modules between the controller module and the loopback module.







simulate this circuit &ndash; Schematic created using CircuitLab


  A 10 digits number would have up to 210 addressable LEDs. Is it a problem?


No.


  For option 2: Which microcontroller is recommended for this setup?


We don't do product recommendations here.

The key question is whether the MCU has all the necessary I/O to communicate with its neighbors (UART?) and also to control all of the LEDs (GPIO).