Yes, the 50% rule applies to apartments. I assume you mean an apartment building, though the way you say that is a little unclear. Owning a "bunch" of anything will put you closer to that number each year. With one, you can get lucky or unlucky and be pretty far off 50% in any particular year. With 50, you're going to be very close, overall.
A duplex could be considered multi-family. For lending purposes, 1-4 units fall into the Fannie Mae type conventional loans, where 5+ requires a commercial loan. For building codes, 1-2 may be under one code (e.g., the International Residential Code) while anything larger falls under a different code. For tax purposes (depreciation), they're all "residential".
Expenses on a poorly operated multi can be much higher, even over 100%. Those are turn around opportunities, if you have the experience and financial ability to do it.