system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If I had more brain cells, would I be smarter?

Or is it something else in the brain that makes people smart?

I assume there is no way to "grow" more brain cells either, is there?
gpt
Potentially smarter, yes. While the brain is very scarcely understood, it's fair enough to say that with better infrastructure, you'd be potentially more capable as a learner.

But if your personality (and other environmental factors) doesn't push you to learn new things, you obviously won't know much more than you do now. Well, why does how much you know matter? The question is how smart you will be, isn't it? The thing is: "smartness" is typically associated with how quickly you learn new information, and learning ability grows as you amass more knowledge of the same kind.  
If by "smart", you just meant the base potentiality, then the amassed body of knowledge probably doesn't matter.

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I don't think you can actually develop more grey matter significantly by any available means, but you can enhance the links between concepts in the theoretical neural network for quicker access to information in general.  
ELI5 version of the previous sentence: Practice makes ~~perfect~~ better, even for thinking.

Future generations have the scifi-esque stem-cell manipulation, but there's little hope for us.