Hi!
If this is a LXC question and not an LXD question (see https://blog.simos.info/comparison-between-lxc-and-lxd/), then you can set lxc.start.auto to 0. See more at https://linuxcontainers.org/lxc/manpages/man1/lxc-autostart.1.html
If instead you use LXD, you can get a list with all your containers with the following command.
lxc list --format=json | jq --raw-output '.[].name'
Then, for each container, you can run
lxc config set mycontainer boot.autostart false