Some years ago, some people said cpus reached their limit, and any improvements were impossible, because of temperature problems
And then we got multiple cores !
But again, what are cpu's limit ??
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Depends on what you define by "limit".
You can put more transistors in a CPU in order to add more features, more cache etc... at the cost of an increased power consumption and the CPU becoming costlier and more complicated to design and to produce.
Also, we are limited by smaller transistor sizes, which add to those complexities (transistors behave in rather complicated ways at low sizes).
Any talk about "limit" is pretty much speculation, it will depend on which technologies are developed in the future.