Hello Brendan,
this post should be reposted under Unified Communications-> IP telephony where you can find VOIP experts.
I can only say that max-conferences 6 means max of 6 contemporary conferences.
the real capabilities depend on the DSP modules that your device has on the board.
if you have PVDM2-x
PVDM2-16 has one C5510 DSP
PVDM2-32 has two C5510 DSP
and so on
you can have a max of 8 conferences, with each one a max of 8 participants on a DSP C5510.
in your case max conferences is set to 6 to leave DSP resources for other usages.
the router is configured with dsp farms and profiles and use SCCP to register DSPs profiles to CUCM.
Hope to help
Giuseppe