Edit systemd service file
$sudo nano /etc/systemd/system/redis.service
Add or update the LimitNOFILE
[Service]
…
User=redis
Group=redis
LimitNOFILE=65536
Once done then you must daemon reload and restart the service
$sudo systemctl daemon-reload
$sudo systemctl restart redis.service
To check if it works, try to cat proc limits
$sudo cat /run/redis/redis-server.pid
$sudo cat /proc/PID/limits