The most official document for the US is Forest Service Trail Accessibility Guidelines.
There is no simple single number for maximum grade. Your are allowed steeper grade for shorter lengths, and the firmness of the treadway also factors into it. It gets complicated. You really need to at least skim over that document. Note that the document above also refers to several other documents. It gets complicated.
All that said, page 10 of the linked document gives a good overview:
But, it's still not that simple. Many state, regional, and local government bodies and private groups owning the land have their own restriction. Sometimes they reference the Forest Service guidelines, with modification. Other times they are stand-alone documents. To get a definitive answer, particularly for not US Forest Service land, you need to do some digging.