I think there is no a concrete number of maximum managed appliances of each type. Because it is highly dependent on many additional factors:
Will there be a separate log server? How many logs gateways would generate daily? What blades will be enabled on the management server and on gateways? Will it be used as a multi-domain server? How many policies would be configured? How many rules and objects would be configured in each policy? And so on.
From the description of the server and of the license you can make a conclusion that 150 enterprise-level gateways can be managed for sure. But again in real life it would depend on all the additional factors.