The paperclip maximizer scenario assumes that recursive self-improvement is possible, which means there will most likely be only a single AI of superhuman power.
There are ancillary assumptions implicit in that (either recursive self improvement and the decision to eliminate humanity is so fast that no counter intelligence uninterested in paperclips can be made, or that how recursive self improvement has been achieved is such a mystery that no counter intelligence uninterested in paperclips can be made. Also that recursive self improvement doesn't itself involve either sequentially or simultaneously coming to adopt a range of views on the value of humans with respect to paperclips)
To be fair, assumptions about single superhuman intelligences make a little more sense if we're talking about a secret Skynet project carried out by a state's most advanced research labs and not a mundane little office supplier's program accidentally achieving the singularity after being tweaked for paperclip output.
I do not follow the “which means”. There are many obvious and hidden variables that will modulate a one-versus-many AGI outcome. Bostrom has a lot on this topic. Couldn’t a true AGI want companionship of peers like we do?
Of course, it is possible that such an AI, on the way to making paperclips, will realise it wants companionship, and even maybe human companionship.
The argument around AI safety is not that it's impossible for a friendly AI to emerge. It's that there are far more ways to build an AI that doesn't care about human life and wipes us out without even thinking about it, than ways to build a friendly AI, and we have no idea which one we're building or how to tell them apart before they're built.
As for the "will there be several AIs fighting each other" hypothesis, that depends on how rapid the exponential take-off is once a self-evolving AI emerges. But a very plausible scenario is that whichever one starts taking off first ends up so far ahead of the others that it is effectively the only game in town and does whatever it wants.
To be fair, assumptions about single superhuman intelligences make a little more sense if we're talking about a secret Skynet project carried out by a state's most advanced research labs and not a mundane little office supplier's program accidentally achieving the singularity after being tweaked for paperclip output.