Never heard of a limit, but I've never had to reserve that many either. The biggest DHCP pool an MX can handle is /19, per vlan, so I don't think 600 reservations over 5 vlans would be too many.
I don't think there is any limitation to the reservation limit. Are you planning to run DHCP on the MX ?
The only issue I see is : Note that the largest DHCP pool the MX will serve is equivalent in size to a /19 subnet, even on a VLAN configured with a larger subnet
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