I am designing a device for home automation but I need a microcontroller with 9 UART channels. Can you recommend any or should I use multiplexer? 
It something strange in your project if it needs so many UARTs. Can you tell more about it?
Teensy 4.1 has the most I beleive, 8 UARTs.
But then the obvious question, why so many UARTS ?
The multiplexer would only be of use if only 1 of your 9 devices transmitted data at once. You would need to know when each device was going to transmit, so that you could have the multiplexer configured in time to route that data. Not so much of an issue in a command-response system.
The "go to" protocol for multiple serial devices is RS-485. Situations like yours, are why it was developed.
Although, you didn't explain your situation fully as the forum guidelines suggest:
https://forum.arduino.cc/t/how-to-get-the-best-out-of-this-forum/679966
The plan is to have 9 seprate uarts that will eventually be converted to RS-485, so I will have 9 seprate RS-485 channels.
A 'home automation' system that needs a least 9 UARTS sounds a bit odd, and limiting.
I mean if there was a microcontroller with 9 UARTS, and that was OK, why would you prefer more ?
And why would you need 9 RS-485 channels, thats very unusual ?
Maybe describe the actual project in a bit more detail ?
Do you need 9? RS485 supports multiple devices as @anon57585045 hinted at in post #5.