As of 9.0, an MXeIII expanded can have up to 1500 total IP+TDM devices (not just phones, all devices includes trunks). Note that it is stated "The total number of IP plus TDM devices should not exceed the value shown with maximum IP devices installed, or 1.5 times the IP limit with fewer IP devices, under typical office traffic conditions." (honestly, I find that statement confusing)
1500 - Maximum devices total
1400 - Maximum IP devices (Minet+SIP devices)
576 - Maximum TDM devices (ONS stations)
I believe the "5600" number comes from how the database itself is configured, that it supports 5600 programmed "devices" which in this case means sets, trunks, groups, etc. not just phones.
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https://www.mitel.com/document-center