x86 is primarily Intel Atom processors , which are installed in a considerable number of real devices, and not emulators. The emulator can just emulate any architecture, regardless of the architecture of the processor on which it is hosted - he and the emulator. You can see this for yourself by looking at the list of images for emulation:

The minimum set of supported architectures is the APP_ABI := x86 armeabi . Next, you should decide whether your application will use the additional capabilities of the armeabi-v7a architecture , for example, NEON - if so, then this architecture should also be included in the list ( extended list of arm-v7 instructions ) - in fact, now almost all ARM processors, at least v7
Recently, devices with processors on a 64-bit architecture are increasingly available, so they should also be taken into account.
You can just ignore mips / mips64
You can find out the architecture of your device by the specification of the processor it uses. For example, Qualcomm Snapdragon 820 - ARM-V8A . Also in the market you can find programs that show system information.
Several ARM architectures in one processor cannot be, but each next one includes the previous one, that is, ARM is included in ARM-V7, ARM-V7 is included in ARM-V8. In fact, the most current ARM platform is now ARM-V7, an absolute majority of existing processors are built on it, but ARM-V8 is actively coming.
The difference in ARM architectures is obvious, each next step is an evolution, new features are added at the level of the iron core.
You can specify to build a library for all abi and this will be a good solution if the final size of the application suits you - each additional platform - new files that are sized.
The official guide on this issue.