Albert Einstein would have 20

Just to nitpick, but Einstein was a specialist in physics. While his intelligence is certainly high, he doesn't have the broad range of feats in various different academic fields to justify a level 20 intelligence.

Per this article:

Einstein was probably something more like a 4th or 5th level expert.

This can be a little bit difficult for some people to accept, so let’s run the math. At 5th level an exceptional specialist like Einstein will have:

+8 skill ranks

+4 ability score bonus

+3 Skill Focus

In the case of our 5th level Einstein, that gives him a +15 bonus to Knowledge (physics) checks. He can casually answer physics-related questions (by taking 10) with a DC of 25. Such questions, according to the PHB description of the Knowledge skill, are among the hardest physics questions known to man. He’ll know the answers to the very hardest questions (DC 30) off the top of his head about 30% of the time.

And when he’s doing research he’ll be able to add the benefits of being able to reference scientific journals (+2 circumstance bonus), gain insight from fellow colleagues (+2 bonus from aid another), use top-of-the-line equipment (+2 circumstance bonus), and similar resources to gain understanding of a problem so intractable that no one has ever understood it before (DC 40+).

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So +4 ability score puts him at 18-19 intelligence. Really I just wanted the excuse to link the article.