If you have the need to nest 5 levels of for-loops, I suggest taking a step back and rethinking your approach, my friend.

Even if that other approach is just refactoring it into separate methods.

I just do i2, i3, etc

Two or three "i"s is readable, but any more and you're counting.
I'Ve started using i, k, m, n that's usually enough.

When you have multiple indices you're also bound to have multiple cardinals those indices count up to, say foo.length and bar.length, so foo_i and bar_i are perfectly legible and self-documenting. A bit Hungarian but Hungarian is good in small amounts. Unless you're dealing with width and height in which case it's x and y but it's not that width_i would be incomprehensible.