What number of bits that can be processed at one time is?

The number of bits that can be processed at one time is known as a processor's Word width (but other names are also used, such as the processor's bus width).

The Word width varies between processor models. Early processors processed 4 bit Words, then 8-bit processors became common. Today's processors tend to support 16, 32 and 64 bit Words. The Word width is not necessarily a power of two; some processors are designed for 24-bit Words, for example.