Why is the maximum % for a gold alloy 92%?
It's not that you can't, it's that you usually don't.

The majority of Gold goes into jewellery with some in coinage (more in the past, we make gold coins now but they don't circulate much). But pure gold is too soft to be useful in most applications (coins have to last, rings have to not scratch too much and they aren't much use if they bend too easily). It is also expensive. So the most used alloys have to make a compromise between usability and cost. There are also some that try to achieve a particular aesthetic effect like a different colour.

This imposes limits of practicality. At the cheap end alloys that look like gold might be mostly other metals. But at the high end the issue is to get the correct aesthetic effect while retaining the appeal and value of gold without having something  too soft or weak to be practical. Many coloured golds are only about 75% gold (18 karat) because that gives a wide range of colours. But top end "pure" gold objects can be made at 22 karat (92% isn). We could make richer alloys, but they would be too soft. 

So, mostly we don't. We can make many alloys with more gold, including many that would be useless as jewellery like the AuBe alloy mentioned in another answer (it is far too brittle), but you will rarely see them as they are not useful for anything.