The limits on the 26-bit Wiegand format are determined by the maximum binary values (0 or 1) that can fit into the bit format. For example, there are 255 possible unique facility codes, because if all 8 of the available facility code bits are set to 1, they equal 255. For ID codes, there are 65,535 possible unique codes, because if all 16 ID code bits are set to 1, they equal 65,535.
26-bit Wiegand is an open format, meaning that the format description and organisation is freely and publicly available for anyone to view and use. As a result, almost all access control systems are compatible with it.
It is important to note that while 26-bit Wiegand is the most common format, there are many others available, with different numbers and formats of bits. The standard 26-bit format is the only one for which the format is dictated by the number of bits (as this format has been standardised). With other bit lengths (34-bit and 37-bit are also used in access control), there may be a wide variety of different formats with the numbers of bits organised in very different ways.