A very small number. If it were 50, then it would be a problem.
This way, you don't have to spend unnecessarily on another MX.
Another option is to create a Linux VM within the network and create tunnels between this machine and the third-party firewall. This way, you would be able to route through the Linux machine and create a route on the HUB and thus guess the route to the SD-WAN.
These are free options.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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