Reserve /dev/ttyUSBx
Yes. You need to create new unique names for them using udev rules.

By using the unique serial number, along with the VID and PID of the board you can use udev to create a symbolic link to a device name of your choice. Best to keep it to something that the Arduino IDE recognises if you want to access the link from within there for programming (otherwise you can choose what you like).

Rules are stored in /etc/udev/rules.d and take the form of .rules files with various instructions in.  One example, that should do what you want, is:

SUBSYSTEMS=="usb", ATTRS{product}=="Arduino Uno", ATTRS{serial}=="6493832323135111C0A1", KERNEL=="ttyACM*", SYMLINK+="ttyACM50", MODE="0666", GROUP="plugdev"


Of course, tweak it to your personal requirements.

Once you have crafted your rules be sure to restart udev: 

sudo systemctl restart udev


or

sudo /etc/init.d/udev restart


and plug your board in.

Genuine Arduino devices usually use /dev/ttyACM* not /dev/ttyUSB*. If you have /dev/ttyUSB* then you are either on a very old device (with an FT232 chip) or a cheap Chinese clone.

If the latter then your mileage may vary somewhat.