system
Given the following post, respond as if you're commenting on Reddit:
human
Have octopuses always had 8 tentacles or gradually increase that number through the course of evolution?

Also, do we know that 8 tentacles is the sweet spot, or could a hypothetical 9-armed cephalopod (novopus?) have a significant advantage in survival and produce many 9-armed offspring?
gpt
Ancestral cephalopods generally had more than 8 appendages. Squids have 10, as do cuttlefish. Nautilus have dozens.

There's probably a trade-off in how many arms you have and how good they individually each are at doing things.  They require dedicated brain space to control all the muscles and suckers and sensory nerves. Natural selection doesn't care about that directly, only about which octopus survives and reproduces best.  So there's probably a variety of selection pressures happening.  Ability to lose a limb and still function.  Ability to catch prey.  Ability to camouflage well.  Ability to impress a mate.

There's also a strong evolutionary pressure against badly breaking their current ability to survive and reproduce.  Maybe, in theory, a two-armed octopus would be much better at all those things \*after a couple centuries of evolution\* learning how to use its fancier tentacles with more dedicated brain space for them.  However, in the short run, a new mutant with only two tentacles probably won't survive to reproduce. It will probably have a hard time catching food, or mating. As such, the traditional 8 armed octopus probably represents a local optimum.  You might be able to make something that works better than their current body plan, but you'd have to badly break things in the short run to get to the new optimum.  Remember, they live in a fiercely competitive environment where everything is trying to eat them or avoid being eaten by them.  They are REALLY good at thriving in their current environment, but even things that slightly compromise that are a serious problem.

Think of it like when you're a giant business that has hundreds of dedicated applications that all run on Windows 95.  Windows 95 sucks ass, it's inefficient.  However, in order to switch to something more modern you would need to essentially break everything and start over. For a business that sucks, but it just costs money and time. For an animal you die and don't reproduce and so you don't really get the chance to test whether the switch is better. In this scenario it would be a business locked in fierce competition with a rival. They can't afford to switch from windows 95 because they will lose all their customers to their rival in the month where all their computers are shut down.