@aalptekin, your topic has been moved to a more suitable location on the forum; see About the Installation & Troubleshooting category.
Some of the newer Arduino boards based on ARM processors might support up to 6 serial interfaces. The processor on e.g. the MKR Zero (SAM D21) does support 6 serial interfaces but I do not know how many are connected to the pins on the board.
Based on your other thread (I did not follow it to the end), a detection of a falling edge might be sufficient to determine the presence of serial data; that would either indicate a start bit or toggling of data bits. For that, any pin should do.