Systemctl service configuration
Start ustreamer at boot, restart, check status. This process is designed to be able to start multiple cameras with one service script using an arguments.

Preparation
Confirm ustreamer is in the path to be used and has necessary permissions:

$ ls -l /usr/bin/ustreamer
-rwxr-xr-x 1 root root 253268 Nov 16 21:18 /usr/bin/ustreamer

Create a system user for better security and not run as root:

$ sudo useradd -r ustreamer

Add the system user to the group video allowing the system user access to the device:

$ sudo usermod -a -G video ustreamer

... or manually edit /etc/group.

systemctl creation
Create new service file with @ in the name to use start arguments:

$ sudo vi /etc/systemd/system/ustreamer@.service

Here is an example. You will need to update the options as needed for your setup and use:

[Unit]
Description=uStreamer service
After=network.target
[Service]
Environment="SCRIPT_ARGS=%I"
User=ustreamer
ExecStart=/usr/bin/ustreamer --process-name-prefix ustreamer-%I --log-level 0 --device /dev/video%I --device-timeout=8  --quality 100 --resolution 1920x1080 --desired-fps=29 --host=0.0.0.0 --port=808%I --static /var/www/html/ustreamer-%I/
[Install]
WantedBy=multi-user.target

Enable the service:

$ sudo systemctl enable ustreamer@.service

Enable the service with argument for starting at boot:

$ sudo systemctl enable ustreamer@0.service

Start the service:

$ sudo systemctl start ustreamer@0.service

The number after the ustreamer@ is the webcam device ex: /dev/video0, /dev/video1, etc. This number can also be used in the other options such as --port, --static, etc.

Verify added service is working:

$ sudo systemctl status ustreamer@0.service
● ustreamer@0.service - uStreamer-0 service
   Loaded: loaded (/etc/systemd/system/ustreamer@.service; disabled; vendor preset: enabled)
   Active: active (running) since Tue 2020-11-17 09:07:49 EST; 9s ago
 Main PID: 9901 (main)
    Tasks: 7 (limit: 4915)
   CGroup: /system.slice/system-ustreamer.slice/ustreamer@0.service
           └─9901 ustreamer-0: /usr/bin/ustreamer --process-name-prefix ustreamer-0 --log-level 0 --device /dev/video0 --device-timeout=8 
--quality 100 --resolution 1920x1080 --desired-fps=29 --host=0.0.0.0 --port=8080 --static /var/wwww/html/ustreamer-%I/

Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39419.673    stream] -- Using TV standard: DEFAULT
Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39419.690    stream] -- Using resolution: 1920x1080
Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39419.690    stream] -- Using pixelformat: YUYV
Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39419.708    stream] -- Using HW FPS: 29 -> 5 (coerced)
Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39419.708    stream] -- Using IO method: MMAP
Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39419.749    stream] -- Requested 5 device buffers, got 5
Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39420.123    stream] -- Capturing started
Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39420.123    stream] -- Using JPEG quality: 100%
Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39420.123    stream] -- Creating pool with 4 workers ...
Nov 17 09:07:55 hostname ustreamer[9901]: -- INFO  [39420.124    stream] -- Capturing ...
lines 1-18/18 (END)

You might want to make sure that each usb port has a specific camera. This is easy to do with udev:

https://unix.stackexchange.com/questions/66901/how-to-bind-usb-device-under-a-static-name

https://wiki.archlinux.org/index.php/Udev#Setting_static_device_names

For example: https://github.com/pikvm/kvmd/blob/master/configs/os/udev/v0-vga-rpi3.rules#L3