Using its own embedded multipoint resource, EX90 is able to make 4 simultaneously calls, either registered to CUCM or not. However, to enable this feature, you need to have the license "Multisite Support" (LIC-EX90-MS).
And yes, this is a hardware limitation, you can have up to 4 720p30 participants connected to your EX90.
Take a look at the datasheet of EX series:
CTRL + F and search for "Multisite".
I hope this help.
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