If there are 3 VMs alive, each of our edge servers gets a “pinned” VM and sends every UDP packet it receives to the same app VM. It’s dumber than you’d think, it literally just sends the packet to the first VM it sees in a sorted list.
Most people run one VM per region for UDP work. So far we haven’t had anyone need to scale out to support giant amounts of UDP traffic, even using the one VM to do protocol / app specific balancing has been enough.