Actually, for compilers, platform is typically not just hardware, but the operating system as well. Windows and Linux are night-and-day different for major platform requirements (e.g., filesystems, I/O), and even Linux and FreeBSD can be important target differences in some platform areas.

So the number of platforms is approximately {x86-32, x86-64, ARM, AArch64}×{Windows, OS X, Linux, iOS, Android} (- a few combos) ~= 17 (I think). MIPS, Sparc, and PowerPC can be useful in some scenarios, as can even x86-16 (hey, wanna write the initial startup code on x86?), and platforms like PNaCl, Emscripten, or WebAssembly are effectively their own platforms as well.