Assuming infinite matter were available, is there a limit to the possible consciousnesses that could be made out of it?
No limit, unless you construct an arbitrary definition of 'consciousness' that for some reason decrees that vast sets of different consciousnesses must be lumped in together as one.
Assuming a speed limit of communication such as light speed, why couldn't sufficiently large minds always either be made from less matter or merely be larger versions of smaller, identical patterns?
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If the resulting discussion becomes impractical to continue here, it means the topic is a promising candidate for its own thread.