You'll be bound by the number of useable display 'outs' on any given GPU, as not all GPUs can use all their display outs at one time.
You can adapt from DisplayPort etc, into HDMI - but even then, getting 8 outputs is not very likely, you'll typically be maxed out at ~4.
DisplayPort (depending on implementation/monitors) can daisy-chain, getting you more displays - but as soon as you adapt them to HDMI, you'd lose that functionality.
So, generally yes, you'll need a multi-GPU setup to push 8 displays - and you'll almost certainly need DisplayPort to HDMI adapters too, as GPUs with 4x HDMI outputs are going to be non-existent (or very, very rare!).
However you don't need to function in SLI or CFx to do so - you should be able to operate mixed brands/models etc to achieve what you want. You just need the multi-GPU component, not for them to strictly work in tandem.
Do you have any GPUs at your disposal already? Or would you be buying new?