About your question

> how many variables can you really remember at once?

and our brain being a bad runtime for programming:

I started programming on a Sinclair ZX81 with 1 kB of RAM. I wrote my program on paper, reasoned about it by tracing variables on paper and eventually I typed in the program on the keyboard and run it. That was much better than typing line numbers and BASIC commands directly into the REPL.

Of course I had bugs and the actual values of those variables surprised me. They still do but I have a much wider view on the software now, usually a couple of 100x50 columns of code side by side. The ability to follow the values is not much better, but values are seldom the important matter, at least in web development, backend and frontend. It's more about the control flow and the data structure. That's more similar to the organizational issues of many other human activities. This goes there, that goes into that other place. My old 1 kB computer programs had little space for organization.