As mentioned MX HA requires unique WAN IPs and reachability to dashboard from each MX.
So you could get multiple IPs from one provider and connect each MX to the provider equipment or use multiple providers with one IP from each and each connected to only one MX. Either way supports HA.
Of course with the second example of two providers only connecting individually to each MX you cannot use a virtual IP for the WAN interface(s). And failover isn't as seamless as failing from the primary to spare MX would mean a new WAN IP is in use.
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