By how many individual voters will the 2024 US presidential election be decided? With that I mean if I would optimally select voters from specific states to not vote, how many would I need to select to change the outcome of the election.

Despite good margins in the electoral college and the popular vote, Biden would have lost if only 43,809 of his voters stayed home (or Trump had this amount more voters).

Here is the smallest change in terms of voters that would have changed to outcome of the 2020 election:

Arizona: 10457
Georgia: 12670

[Edit Nevada: 33596 isn't needed, since tie is enough]
Wisconsin: 20682

--> Total voter margin: 43809

The 2016 election was decided by 80,000 votes. Source for both

In case of a tie in the electoral college, the vote would go to the (incoming) house of representatives (to keep things simple, I am ignoring the vice president).
[Edit after commenter Calibrate: The House delegation from each state casts one vote. So it depends on how many states each party wins in terms of house delegates. If the delegates cannot agree (e.g. in case of a tie), the delegation cannot cast a vote, see ballotpedia.] For the 2020 election this meant that a tie would have allowed Trump to win.