Do a `ls -l /dev/nvidia-uvm` on the host. It shows major and minor device number right after the GID column.
In the lxc-config I have e.g.
```
lxc.cgroup2.devices.allow = c 236:1 rwm
lxc.cgroup2.devices.allow = c 236:0 rwm
...
lxc.mount.entry = /dev/nvidia-uvm-tools dev/nvidia-uvm-tools none bind,optional,create=file 0 2
lxc.mount.entry = /dev/nvidia-uvm dev/nvidia-uvm none bind,optional,create=file 0 2
...
```