my plan for now is to deploy 2 instances of fleet with the same mysql backend, one in dmz for devices that runs also outside the company network and one internally for in office devices.

The DB and server load vary quite a bit by how you set the intervals and how many/what queries you schedule. As a start I think you'll find a medium to large cloud DB and a couple of servers able to handle this. Some folks run multiple servers behind a load balancer for high availability, but osquery buffers results if the server goes down temporarily so I don't personally see HA as a hard requirement.

I already have the cluster DB, with HA and LB. My concern is for fleet. Is it better to have multiple fleet instances and do the LB via DNS and/or nginx?